I just use the original humour colors, but with a twist:
SP - Sanguine - blood - red
SJ - Melancholic - black bile
NT - Choleric - yellow bile
NF - Phlegmatic - green
This reverses Keirsey's NF/NT assignments to the humours. But the NT's need for mastery and competence is more traditionally Choleric, while the NF's diplomacy and even its people-focus is traditionally Phlegmatic. (Keirsey judged it only by "enthusiasm" as opposed to "coolness", but the familiar "loud, hot-headed" aspect of choleric is best matched by the In Charge interaction style, rather than an action temperament. So Interaction + Conation are actually two four temperament systems mapped to the 16 types).