Color.elm

module Main exposing (..)

import Html exposing (text)
import String
import Char

toHex : Int -> String
toHex n =
    let
        hex = String.toUpper(toRadix n)
    in
        if String.length hex == 1 then
            "0" ++  hex
        else
             hex

toRadix : Int -> String
toRadix n =
    let
        getChr c =
            if c < 10 then
                String.fromInt c
            else
                String.fromChar <| Char.fromCode (87 + c)
    in
        if n < 16 then
            getChr n
        else
            (toRadix (n // 16)) ++ (getChr (modBy 16 n))

type Color
    = Red
    | Green
    | Blue
    | Rgb { r: Int, g: Int, b: Int }

colorToHex : Color -> String
colorToHex color =
  case color of
      Red   ->
          "#FF0000"
      Green ->
          "#00FF00"
      Blue  ->
          "#0000FF"
      Rgb {r, g, b} ->
          "#" ++ (toHex r) ++ (toHex g) ++ (toHex b)

main = text(colorToHex(Rgb{r = 255, g = 255, b = 255}))

Color.jl

@enum ColorBase red blue green
struct Rgb
 r::UInt8
 g::UInt8
 b::UInt8
end
Color = Union{ColorBase,Rgb}

function to_s(c::Color)::String
  if c == red
    return "#FF0000"
  elseif c == blue
    return "#00FF00"
  elseif c == green
    return "#0000FF"
  elseif typeof(c) == Rgb
    hex = (x) -> uppercase(string(x, base=16, pad=2))
    return string("#", hex(c.r), hex(c.g), hex(c.b))
  end
end

println(to_s(Rgb(255, 255, 255)))
println(to_s(red))
println(to_s(blue))
println(to_s(green))

Color.kk

type color {
  Red
  Green
  Blue
  Rgb( r : int, g : int, b: int )
}

fun to_s(c: color) {
  match(c) {
    Red ->   "#FF0000"
    Green -> "#00FF00"
    Blue -> "#0000FF"
    Rgb(r,g,b) -> "#"+showHex(r,2)+showHex(g,2)+showHex(b,2)
  }
}

fun main() {
  c = Rgb(255,1,255)
  println(to_s(c))
}

Color.re

type color = Red | Green | Blue | Rgb({r: int, g: int, b: int})
let numToHex = i =>
  "00" ++
  Js.Int.toStringWithRadix(i, ~radix=16)
  ->Js.String2.slice(~from=-2, ~to_=Js.Int.max)
  ->Js.String.toUpperCase
let toHex = color =>
  switch color {
  | Red => "#FF000"
  | Green => "#00FF00"
  | Blue => "#0000FF"
  | Rgb({r, g, b}) => "#" ++ numToHex(r) ++ numToHex(g) ++ numToHex(b)
  }

let white = Rgb({r: 255, g: 255, b: 255})
toHex(white) |> Js.log

Color.swift

enum Color {
  case Red, Green, Blue, Rgb(r:Int, g:Int, b:Int)
  func str() -> String {
    switch self {
        case .Red:
            return "#FF0000"
        case .Green:
            return "#00FF00"
        case .Blue:
            return "#0000FF"
        case let .Rgb(r, g, b):
            return String(format:"#%02X%02X%02X", r, g, b)
    }
  }
}

let c:Color = Color.Rgb(r:255, g:255, b:255)
print(c.str())