How come no matter what colour the liquid is the froth is always white? .?

that's a good question. i want to say oxidation or something like that

because it's milk.....lol

Because the color of the natural light is white. What we actually see is the reflection of the natural light in the bubble, which is white.

You haven't seen the froth of a Diet Coke, then.  The froth of a Coke isn't white, either, but it's near white, a very light beige.  The froth of a Diet Coke, though, is dark khaki at best.

I imagine the reason a liquid's color is lightened when it froths is air gets mixed in, which is colorless and transluscent.  The liquid content of the froth being far less than when it is not mixed with air means the elements that refract the liquid's coloring to your eyes are in smaller abundance, too, and the air allowing light to pass through it means that the coloring is mixed appears whiter since light itself is white.