I got that image from the layer name in your screenshot. Grayscale with transparency, a png. Use Image -> Mode -> RGB first. (A tip: If you get a chance of the equivalent svg vector, get that and use inkscape for the fill.)
For that image, not too difficult to colour in, do not expect other more complicated shapes to be easy.
Lots of clicking to make selections required. Use fuzzy select in addition mode with a zero threshold. General advice, use as many layers as required, keep everything separate. Easier to fix if you make a mistake.
I get this as a first shot. https://youtu.be/q6PZNTT-rQk duration 3 minutes
That could be a basis for more colours, more blending.
For that image, not too difficult to colour in, do not expect other more complicated shapes to be easy.
Lots of clicking to make selections required. Use fuzzy select in addition mode with a zero threshold. General advice, use as many layers as required, keep everything separate. Easier to fix if you make a mistake.
I get this as a first shot. https://youtu.be/q6PZNTT-rQk duration 3 minutes
That could be a basis for more colours, more blending.