You have to give the image the setcolor effect (under effects, then the 2nd group down).
I think it may not work on every platform as it's a WebGL effect. But works very nicely on PC's.
I'm playing around currently with a simple verticle shooter and I'm using the smoke effect again. To be safe, I'm just recolouring the smoke in a paint package (a white, red, green, blue and yellow version) and using 4 over the top of each other to get different effects. It's not as good as setcolor, for example to get purple you can use 2 red and 2 blue layers, but it doesn't look purple, it just looks like a reddy/bluey smoke :p