It varies a bit from LED strip to LED strip, typically they are RGB (other times BGR) or any combination thereof.
So assuming that the LEDs are RGB, your first slice is the Red component of the first pixel, the next slice the Green component of the first pixel, the next slice is the Blue component of the first pixel and the next slice is the Red component of the second pixel and so on.
All this assumes that you have set your LED controller so that the pixels start from the first DMX channel.
then there is of course pixel mapping and and all that, but if you stick with one DMX universe (512 channels) it is this straight forward.
see attached patch: ColorsViaArtNet.v4p (10.5 KB)