It’s frequently used to describe the problem of calling an async function from a sync one or vice versa. There’s other applications of the term though (pure vs impure functions).
I think it just happens to be that we like assigning colors to differentiate things in the mathematical side of CS.
From a quick google it’s about the “color” of a function being red or blue as defined here (I think): https://journal.stuffwithstuff.com/2015/02/01/what-color-is-...
Isn’t this weird and unnecessary? I dunno. It’s probably just me.