"coloring" goes back before a few blog posts, it was probably popularized with programmers that keep up with crypto, as 'colored coins' are described in an influential paper right around the creation of Ethereum (Vitalik is one of the authors)[0], but I found some earlier references too, like coloring a complex plot to add a dimension [1], so carrying it over to computer programming, you have functions that have otherwise identical signature that you need to distinguish, so you make them different colors.

[0] https://docs.google.com/a/buterin.com/document/d/1AnkP_cVZTC...!

[1] https://en.wikipedia.org/wiki/Domain_coloring