According to the "kosher color scheme" meat utensils are marked red, and dairy (milchig) is marked blue. Now, red for meat is quite understandable since meat is in fact red. But why should dairy be marked blue? How/when did this custom become established, and is there any record of it (or similar color scheme associations) historically? Does anyone know any other variations of this tradition widespread in certain communities or families?
I'm not sure what you mean by variations, but the Ramma in Yoreh Deah 89:4 does mention the minhag to have two separate knives, one for dairy and one for meat, and to make a mark on the dairy knife. He adds a warning not to deviate from the minhag.
כבר נהגו כל ישראל להיות להם שני סכינים ולרשום אחד מהם שיהא לו היכר ונהגו לרשום של חלב ואין לשנות מנהג של ישראל:
Although like other users have suggested this Minhag is not officially established, that is the case early on with most Minhagim. I think most can confirm that when these are separated by colored marks the color scheme is in fact red meat blue dairy. I suggest that red was the obvious choice for meat and it’s opposite is generally blue so dairy adopted blue despite it having zero relevance to dairy. It could’ve been accomplished with black/white with white resembling dairy but I think those are commonly the colors of kitchen utensils and would not be as easily distinguished.