Vertex painting only outputs red channel ?

I’m trying to make a glass material with a dirt layer. I wanted to make the dirt paintable using vertex color and alpha. I have managed to get the vertex painted alpha to drive the opacty / blending of my dirt layer, but i’m getting stuck on using vertex color to drive the dirt’s color. When I preview my vertex painting node, it seems to only output this 1,0,0 value that seemingly comes from nowhere and will not change whatever I do. I suspect this might be why I can’t get my dirt’s colour to work. Previously I have managed to get it to sort of work, but it seemed like only some colors showed up from the vertex color on the dirt. The blue channel was pretty much entirely missing. Is there something I should know about this ? Is this working properly in 4.20.1 ?
Thanks in advance