Well, in my clinic different colored needles signifies different needle sizes. I went from green (16 gauge) to blue (15 gauge) and now they want me to use white needles which is 14 gauge. The white ones scare me a little...
I only have 40mm between my A and V buttonholes; I don't think I could take larger needles than the 16 gauge I'm using now because of the back circulation.