One way that I can think of doing this is with the marquee effect. You wouldn't need to do anything special in xLights or the F16V2.
Let's say you want your lights to go like this: R G W R G W R G W... but you only wanted to light up the red ones. Just drop a Marquee effect on to the strand and then set the settings so that only the 1st, 4th, 7th, etc. lights are on. Set the color for red and then only the 1st, 4th, 7th, etc. lights will be lit up red. If you then wanted to do green (starting with the second light) you would just have to change the color and the start position. If you wanted them all on as R G W R G W R G W... just use the same marquee effect and select all 3 of those colors. You'll want to set the speed for the marquee effect to 0 so that it doesn't chase.
I have attached the effect settings for the marquee effect.