How I choose the right colors for my drawings?

Regarding your question as how I choose the right colors for my drawings, I use my app Color Picker for Artists, which suggests colored pencils for the color picked from the reference photo. There is also an application for graphite pencils in this app. No ads. If you want to buy it for yourself, go to Apple app store:
Get it for yourself on Google App Store: https://play.google.com/store/....apps/details?id=colo
Apple Appstore: https://itunes.apple.com/us/ap....p/color-picker-for-a
Desktop version for Windows: https://www.pen-pick.com/