Here’s how to paint under a black and white adjustment layer without messing up your colours. If you create a Luminosity layer, you can paint normally while the adjustment layer is still set to Visible. Then, when you hide the adjustment layer, the colours still work with correct temperatures. This enables you to work as if the image were greyscale at any stage of the process.
Filters and colour adjustments
Right at the end of a painting I’ll use adjustment layers such as photo filters and the Curves tool. I find this tweaking phase very enjoyable, and try not to rush it as I search for the perfect mood. Why don’t you try layering different effects, use big soft brushes on layer mode types you’ve never tried out, and get those colours just right!