Transguild Courier appears as a riddle concerning to what color it is.No colored mana cost, it doesn't have the rules text it used to but doesn't have it's Color Indicator either.
Color Indicator: White, Blue, Black, Red, Green
Although in oracle the color indicator text is after the flavor text which manalink/shandalar don't support I guess, originally it had it in it's rules text:
Transguild Courier is all colors (even if this card isn't in play). Can't we just put the color indicator in the rules text?
As explained in it's rulings:
6/7/2013 Although originally printed with a characteristic-defining ability that defined its color, this card now has a color indicator. This color indicator can’t be affected by text-changing effects (such as the one created by
Crystal Spray), although color-changing effects can still overwrite it.