Hi
Vertex painting is a 0-1 scale, so black, white, and gray are typically your best bet for getting specific results in the “paint color” section. The way you have it set up unfortunately will not produce the results you are looking for as it is registering a value other than 0-1. However, if you check the red and green channels but set the paint color to blue as you have, it would paint in the blue channel as it would register as (0,0,1). We have a lot of information on this subject here: Mesh Paint Mode in Unreal Engine | Unreal Engine 5.1 Documentation