Dictionary Wrapper for Custom Types

I'm trying to implement a dictionary wrapper that uses a custom value type like so:

import Foundation

enum Color {
    case Red
    case Green
    case Blue
}

struct IntColorDict {
    private var dict: [Int: Color]

    init(_ dict: [Int: Color]) {
        self.dict = dict
    }
}

extension IntColorDict: SequenceType {
    typealias Generator = IntColorDictGenerator

    func generate() -> Generator {
        return IntColorDictGenerator(self)
    }
}

struct IntColorDictGenerator: GeneratorType {
    typealias Element = (Int, Color)

    var generator: DictionaryGenerator<Int, Color>

    init(_ colorDict: IntColorDict) {
        generator = colorDict.dict.generate()
    }

    mutating func next() -> Element? {
        return generator.next()
    }
}

If I now do

let c: [Int: Color] = [1: .Red, 2: .Green, 3: .Blue]
for (k, v) in IntColorDict(c) {
    print("k: \(k), v: \(v)")
}

the compiler fails with the error

fatal error: value failed to bridge from Swift type to a Objective-C type

The problem obviously is the Color type, since when I replace all occurrences of Color with another Int (i.e. var dict: [Int: Int]) everything works fine. How can I fix this?

Or maybe there is even a completely different solution? What I actually need is a dictionary with a certain value type that has some extended functionality compared to default Swift Dictionaries (but only for that specific value type!). I cannot solve this problem by extending the Dictionary definition as extensions are not supported for data types with "where" constraints, so the wrapper above seems like a natural solution.