Here is a quick GIMP sketch for orientation. The most important planes to shade are those facing downwards. For both colors purple makes a good shadow color.
Make sure to use thin glazes to blend the shadows in. If you have more squigs to paint like this you can also add the shadows with the airbrush after you laid down your base colors. Thin the shadow color down so it builds up slowly. This way you have more control and the blends with airbrush will be super smooth.
I also added some highlights, for example on the chin to pronounce the edge and add more definition to the shape.