Question

Adding a last dash of blue, the painting Ryan had been working on for the three weeks was finished

Answer 1

Help me fix the modifier-subject mismatches

Answer 2

Adding a last dash of blue, the painting was finished on which Ryan had been working for three weeks.

Answer 3

NO, don't add the on it doesn't sound right.
Ryan finished the painting he had been working on for the three weeks after he added the last dash of blue.
Try that.