Painting model only shows Pinks or mix of pinks

This is not an issue with missing textures, the correct texture is applied.

When I’m applying paint to my model in Blender’s painting mode, it applies the colour to the model correctly from the UV Map, but painting in white only shows bright pink, and any darker colours shows shades of dark blue, red, or black, regardless of which colour is selected, as if my selected colour is mixing with the bright pink.

The base canvas colours I’ve tested have been green, white, and magenta, with new files used too. Painting is not showing the colour I’m actually selecting. What’s wrong with this?

image for example, I have painted a strip of the rainbow colours (ROYGBIV) and written a few colours out in their actual colour from the palette, to show how they look on the UV:
https://imgur.com/a/NjOMv42

DIfficult to say without a blend file. But quite odd indeed. Maybe some issue with the color model.