您无需将其强制转换为 NSString 即可使用 stringByReplacingCharactersInRange，您只需更改创建字符串范围的方式，如下所示:

You don't need to cast it to NSString to use stringByReplacingCharactersInRange you just need to change the way you create your string range as follow:

更新:Xcode 7.2 • Swift 2.1.1

let binaryColor = "000"
let resultString = binaryColor.stringByReplacingCharactersInRange(
    Range(start: binaryColor.startIndex, end: binaryColor.startIndex.advancedBy(1)),
    withString: "1")

或者干脆

let resultString2 = binaryColor.stringByReplacingCharactersInRange(
    binaryColor.startIndex..<binaryColor.startIndex.advancedBy(1),
    withString: "1")