Consider an object which responds to two different signals A and B: it responds to signal A with blue color and signal B with red color. Also, it responds only to the signal that reached first. In case they both reach together, there is no observation.
Now consider three observers $x,y,\,z$. If event that both signals reaches at the same time for $x$, then for $y$ and $z$ there must exist velocity $v_1$ and $v_2$ such that these signals aren't simultaneous to them. Let for $y$ A reached first and for $z$ B reached first. Then what will be the actual color the object emits?