Step-by-step (Patreon)

Colouring was done on a single layer, using the attached brush, and the blend too.

More colouring! This time mostly on new layers above the base colouring layer. The same brush was used (though I changed the opacity as needed - it would have been darker here) along with the blend tool. Also used the levels tool to increase the contrast.

More colouring. Mostly on a new layer with the attached brush, but with opacity set to 100% and 'mix ground colour' unticked.

Also set a new layer above everything, set it to overlay, and very lightly airbrushed some oranges/blues/purples over areas that I wanted to have more vivid colours.

And done! I added a lazy background using one of the DAUB watercolour brushes, added a few more details, increased the contrast a bit more using adjustment layers/overlay layers. I also set an overlay layer to 19% opacity, filled it with a medium grey, and then used the perlin noise filter with the below settings to add a bit of texture.