Just a tip learnt from exprience. If you are kntting a garment that uses one hank, skein or my personal favourite - shank, ( hank + skein = shank) make sure you blend them together. If you don't you will often wind up with a sudden colour change.
The experience part? One jumper dark on the back and lighter on the front and an odds and evens coat which is half light purple and half dark. Of course I must have done these on purpose because they make such good examples to show people at the markets we sometimes do.
To avoid colour changes, wind your "shanks" into balls. Knit 2 rows from one ball and then two rows from another. Before I wind them into balls though, I put them on the floor and have a good look at them. I pick out the two that are the most different and make sure I knit with these two first. This will even them out.
Why two rows? Give it a try and you will find out.
This also explains why we have 300gm balls of yarn for sale. Many of our patterns use 300gms of yarn and it is really hard to mix three shanks together.