Besides the DOTS you will need a random number generator (6 sided dice), a 1x6 plate for a reference, a brick separator and a studded surface to decorate. On the 1x6 plate, leave the first 3 studs empty and then put one tile of each of your 3 chosen colours on the remainder.

Choose how big your Aliens will be (e.g. 6x6), then for each stud in the left half of the grid roll the dice.and count along your reference. Numbers 1-3 will leave the stud empty and 4-6 will select a coloured tile to place.
Once you have filled the left half, mirror the design onto the right. At this point I like to choose the shapes so every tile is touching by at least a corner. I might also fill in a few empty studs by randomising a colour, to get it looking correct.

The method for generating the aliens is given in this python article. My implementation to create LDraw files is available on GitHub.