Well, there must be a solid reason as to why Blizzard did not allow the illusion to appear none other than the pre-set colour (Blue) to allied units - to avoid confusion.
As for Tropy, the only thing I can think of is that you did not really create an "illusion" but somehow a copy of that Hero is being created on that game - such as Meepo from DotA, they have the same colour because they are indeed 4 different units.