I tried the iOS native drawing app freeform. It's limited, but easy to use (when it doesn't crash too often), it can come in handy. Here are some sketches I made for fun.

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When using freeform, you can lock a doodle your use as sketch reference, so it’s easy to separate from inking. While paint can be added while selecting inked layer, so you can paint bucket (color shapes behind ink).

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