Color.operator -  

// magenta - blue = red
var redColor : Color = Color.magenta - Color.blue;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Color redColor = Color.magenta - Color.blue;
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public redColor as Color = (Color.magenta - Color.blue)