Swift Image from Color. Supports both UIImage and NSImage for cross-platform compatibility.

Image+Color.swift

#if os(iOS)
import UIKit
public typealias PlatformImage = UIImage
public typealias PlatformColor = UIColor
#else
import AppKit
public typealias PlatformImage = NSImage
public typealias PlatformColor = NSColor
#endif

extension PlatformImage {

    public convenience init(color: PlatformColor) {
        let size = CGSize(width: 1, height: 1)

        #if os(iOS)
        let image = UIGraphicsImageRenderer(size: size).image { context in
            color.setFill()
            context.fill(context.format.bounds)
        }.resizableImage(withCapInsets: .zero)
        self.init(cgImage: image.cgImage!)
        #else
        self.init(size: size, flipped: false) { rect in
            color.setFill()
            rect.fill()
            return true
        }

        resizingMode = .stretch
        capInsets = NSEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
        #endif
    }

}

Example usage. Can be pasted directly into a playground with the above code.

let image = PlatformImage(color: .red) // optionally you can use UIImage directly if you only need iOS support.
let frame = CGRect(x: 0, y: 0, width: 40, height: 40)

#if os(iOS)
typealias ImageView = UIImageView
#else
typealias ImageView = NSImageView
#endif

let imageView = ImageView(frame: frame)

#if os(OSX)
imageView.imageScaling = .scaleProportionallyUpOrDown
#endif

imageView.image = image