What is a `red vertex` and what is a `blue vertex`?

Question

I showed the following question on an exam:

let $G(V, E)$ connected indirected graph with positive weights.

any vertex is colored with either blue or red.

Claim: if edge $(u, v)$ is the lightest from the edges $(x, y)$ that x has difference colors from y (for exaple, x is red and y is blue),

Then there is MST of $G(V, E)$ that contains $(u, v)$.

My question is, what is a blue vertext and what is a red vertex?

I know only about red and blue edges. thanks !

Answer 1

I'd have written "every vertex is colored red or blue" rather than "any . . .".

It just means that they're dividing the set of all vertices into two disjoint subsets and calling the members of one set "red" and those of the other "blue".