Thank you!! I have decided that after fiddling with the settings, Medibang's airbrush is actually acceptable for shading (and is also my only option, as I still hate cel-shading or soft cel-shading in Medibang).
About the colour scheme - I actually did start out with orange clothes, purple-hued metal, and pale green wherever the turquoise is now, but I changed the flat colours because they looked horrid But I will make a secondary colour scheme work one day with unmistakable secondary colours! I will put forest green with amethyst purple and the absolutely obnoxious orange of the fruit (have you ever noticed how ridiculously vivid oranges are?!) and have it not turn out terrible!