An easy way to separate your colours is to pre-twist each individual colour before spinning them all together.
For example, if you want to make a type 6 string with 3 separate colours, you would twist 2 loops of each colour just enough for them to stay together, and then spin them all together as normal.
You can also pre-twist different colours/types of thread to blend them together evenly.
@smileypants707 goes over pre-twisting and everything else you’d ever need to know about making strings in his string making mega-thread. Definitely worth reading if you haven’t already!