The first tedious step involves using the Lasso tool to mark out flat areas of colour for different elements on separate layers. Having them on separate layers is key because I can easily change the colour of, say, the soup without affecting the ginger pieces or noodles - and I can also turn the layer contents into masks. This flexibility is why I prefer to work digitally when it comes to colouring.
I then go in with a brush to add lines, marks, and texture. In fact, for this piece, there are only three brushes I use - one from Kyle Webster's watercolour pack, another for the rough texture, and the basic round brush for blocking in colours. You don't need many fancy brushes once you find a core set that you're comfortable using. Plus, the gradient tool set to low opacity is a great way to add subtle changes in hue or value quickly.
After that, I make decisions as to what colours I want the lines to be, keeping in mind that the central circle is the focus of the piece and that the DNA strands or ingredients on the periphery shouldn't stand out too much at first. In this close-up you can see that the lines for the ingredients have been given a china blue overlay, while the ones on the noodles are a darker maroon.