Convert iTerm2 color scheme to Terminal.app color scheme. To use, just save to disk and run it like you would any script. NOTE: Needs Xcode 8.0 or later, since it has been ported to Swift 3.0.

iTermColorsToTerminalColors.swift

#!/usr/bin/xcrun swift
//
// Copyright © 2016 Leon Breedt
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.


import Cocoa

/// Enumerates the colors in an iTerm color scheme.
enum iTermColors: String {

    case ansi0 = "Ansi 0 Color"
    case ansi1 = "Ansi 1 Color"
    case ansi2 = "Ansi 2 Color"
    case ansi3 = "Ansi 3 Color"
    case ansi4 = "Ansi 4 Color"
    case ansi5 = "Ansi 5 Color"
    case ansi6 = "Ansi 6 Color"
    case ansi7 = "Ansi 7 Color"
    case ansi8 = "Ansi 8 Color"
    case ansi9 = "Ansi 9 Color"
    case ansi10 = "Ansi 10 Color"
    case ansi11 = "Ansi 11 Color"
    case ansi12 = "Ansi 12 Color"
    case ansi13 = "Ansi 13 Color"
    case ansi14 = "Ansi 14 Color"
    case ansi15 = "Ansi 15 Color"
    case cursorText = "Cursor Text Color"
    case selectedText = "Selected Text Color"
    case foreground = "Foreground Color"
    case background = "Background Color"
    case bold = "Bold Color"
    case selection = "Selection Color"
    case cursor = "Cursor Color"

}

/// Enumerates the colors in a Terminal.app color scheme.
enum TerminalColors: String {

    case ansiBlack           = "ANSIBlackColor"
    case ansiRed             = "ANSIRedColor"
    case ansiGreen           = "ANSIGreenColor"
    case ansiYellow          = "ANSIYellowColor"
    case ansiBlue            = "ANSIBlueColor"
    case ansiMagenta         = "ANSIMagentaColor"
    case ansiCyan            = "ANSICyanColor"
    case ansiWhite           = "ANSIWhiteColor"
    case ansiBrightBlack     = "ANSIBrightBlackColor"
    case ansiBrightRed       = "ANSIBrightRedColor"
    case ansiBrightGreen     = "ANSIBrightGreenColor"
    case ansiBrightYellow    = "ANSIBrightYellowColor"
    case ansiBrightBlue      = "ANSIBrightBlueColor"
    case ansiBrightMagenta   = "ANSIBrightMagentaColor"
    case ansiBrightCyan      = "ANSIBrightCyanColor"
    case ansiBrightWhite     = "ANSIBrightWhiteColor"
    case background          = "BackgroundColor"
    case text                = "TextColor"
    case boldText            = "BoldTextColor"
    case selection           = "SelectionColor"
    case cursor              = "CursorColor"

}

// Mapping of iTerm colors onto corresponding Terminal.app colors.
let iTermColor2TerminalColor = [
    iTermColors.ansi0: TerminalColors.ansiBlack,
    iTermColors.ansi1: TerminalColors.ansiRed,
    iTermColors.ansi2: TerminalColors.ansiGreen,
    iTermColors.ansi3: TerminalColors.ansiYellow,
    iTermColors.ansi4: TerminalColors.ansiBlue,
    iTermColors.ansi5: TerminalColors.ansiMagenta,
    iTermColors.ansi6: TerminalColors.ansiCyan,
    iTermColors.ansi7: TerminalColors.ansiWhite,
    iTermColors.ansi8: TerminalColors.ansiBrightBlack,
    iTermColors.ansi9: TerminalColors.ansiBrightRed,
    iTermColors.ansi10: TerminalColors.ansiBrightGreen,
    iTermColors.ansi11: TerminalColors.ansiBrightYellow,
    iTermColors.ansi12: TerminalColors.ansiBrightBlue,
    iTermColors.ansi13: TerminalColors.ansiBrightMagenta,
    iTermColors.ansi14: TerminalColors.ansiBrightCyan,
    iTermColors.ansi15: TerminalColors.ansiBrightWhite,
    iTermColors.background: TerminalColors.background,
    iTermColors.foreground: TerminalColors.text,
    iTermColors.selection: TerminalColors.selection,
    iTermColors.bold: TerminalColors.boldText,
    iTermColors.cursor: TerminalColors.cursor,
]

/// Enumerates the names of iTerm color components in the scheme dictionary.
struct iTermColorComponent {

    static let red = "Red Component"
    static let green = "Green Component"
    static let blue = "Blue Component"

}

/// Converts an iTerm color scheme file (.itermcolors), into a Terminal.app color scheme
/// file (.terminal).
func convertToTerminalColors(itermFile: String, terminalFile: String) {
    if let itermScheme = NSDictionary(contentsOfFile: itermFile) {
        print("converting \(itermFile) -> \(terminalFile)")
        let name = terminalFile.ns.lastPathComponent.ns.deletingPathExtension
        let terminalScheme = convert(iTermColorScheme: itermScheme, toTerminalSchemeWithName: name)
        terminalScheme.write(toFile: terminalFile, atomically: true)
    } else {
        print("unable to load \(itermFile)")
    }
}

/// Converts an iTerm color scheme dictionary into a Terminal.app color scheme dictionary.
func convert(iTermColorScheme: NSDictionary, toTerminalSchemeWithName name: String) -> NSDictionary {
    var terminalColorScheme: [String: AnyObject] = [
        "name" : name,
        "type" : "Window Settings",
        "ProfileCurrentVersion" : 2.04,
        "columnCount": 90,
        "rowCount": 50
    ]
    
    // SF Mono is pretty sweet.
    if let font = archivedFontWithName(name: "SF Mono Regular", size: 13) {
        terminalColorScheme["Font"] = font
    }
    
    for (rawKey, rawValue) in iTermColorScheme {
        guard let name = rawKey as? String else { continue }
        guard let itermDict = rawValue as? NSDictionary else { continue }
        guard let itermKey = iTermColors(rawValue: name) else { continue }
        guard let terminalKey = iTermColor2TerminalColor[itermKey] else { continue }
        
        let (r, g, b) = (
            (itermDict[iTermColorComponent.red] as? CGFloat) ?? CGFloat(0),
            (itermDict[iTermColorComponent.green] as? CGFloat) ?? CGFloat(0),
            (itermDict[iTermColorComponent.blue] as? CGFloat) ?? CGFloat(0)
        )
        
        let color = NSColor(deviceRed: r, green: g, blue: b, alpha: 1)
        let data = NSKeyedArchiver.archivedData(withRootObject: color)
        
        terminalColorScheme[terminalKey.rawValue] = data
    }
    
    return terminalColorScheme
}

/// Creates an `NSData` representation of an `NSFont`.
func archivedFontWithName(name: String, size: CGFloat) -> NSData? {
    if let font = NSFont(name: name, size: size) {
        return NSKeyedArchiver.archivedData(withRootObject: font)
    }
    return nil
}

extension String {

    /// Gets the canonical version of a path.
    var fullPath: String {
        get {
            let path = ns.standardizingPath
            var directory = ns.deletingLastPathComponent
            if directory.utf8.count == 0 {
                directory = FileManager.default().currentDirectoryPath
            }
            return directory.ns.appendingPathComponent(path)
        }
    }
    
    /// Convenience property for accessing this string as an `NSString`.
    var ns: NSString {
        return self as NSString
    }

}

// Entry point.
let args = Process.arguments.dropFirst()
if args.count > 0 {
    for itermFile in args {
        let path = itermFile.fullPath
        let folder = path.ns.deletingLastPathComponent
        let schemeName = path.ns.lastPathComponent.ns.deletingPathExtension
        let terminalPath = "\(folder)/\(schemeName).terminal"
        convertToTerminalColors(itermFile: path, terminalFile: terminalPath)
    }
} else {
    print("usage: iTermColorsToTerminalColors FILE.itermcolors [...]")
}

I ran these commands:

$ sudo xcode-select -s /Applications/Xcode-beta.app/Contents/Developer
xcode-select: error: invalid developer directory '/Applications/Xcode-beta.app/Contents/Developer'
$ chmod +x iTermColorsToTerminalColors.swift
$ ./iTermColorsToTerminalColors.swift Untitled.itermcolors

(I'm not sure what the Xcode error is about)

And at the end I get this error:

./iTermColorsToTerminalColors.swift:188:12: error: instance member 'arguments' cannot be used on type 'Process'
let args = Process.arguments.dropFirst()
           ^~~~~~~ ~~~~~~~~~
./iTermColorsToTerminalColors.swift:174:48: error: cannot call value of non-function type 'FileManager'
                directory = FileManager.default().currentDirectoryPath
                            ~~~~~~~~~~~~~~~~~~~^~

./iTermColorsToTerminalColors.swift:125:18: error: value of type 'String' does not conform to expected dictionary value type 'AnyObject'
        "name" : name,
                 ^~~~
                      as AnyObject
./iTermColorsToTerminalColors.swift:152:53: error: cannot assign value of type 'Data' to type 'AnyObject?'
        terminalColorScheme[terminalKey.rawValue] = data
                                                    ^~~~
                                                         as AnyObject
./iTermColorsToTerminalColors.swift:155:12: error: cannot convert return expression of type '[String : AnyObject]' to return type 'NSDictionary'
    return terminalColorScheme
           ^~~~~~~~~~~~~~~~~~~
                               as NSDictionary
./iTermColorsToTerminalColors.swift:161:32: error: cannot convert return expression of type 'Data' to return type 'NSData?'
        return NSKeyedArchiver.archivedData(withRootObject: font)
               ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                                  as NSData

My

$ swift --version
Apple Swift version 4.0.2 (swiftlang-900.0.69.2 clang-900.0.38)
Target: x86_64-apple-macosx10.9

I ported it to work with Swift 5 (without knowing Swift, so it may be non good code-wise)