How to turn grey parts of a scan white in Painter 2019

Hi all - I do traditional work and edit it in Corel painter. 

I have the 2019 version and can't seem to find a quick way to fix the grey parts of my scanned work. Atm what I'm doing is just bucket filling all the grey parts, but it's very time consuming for detailed work and some parts of the drawings, when using the paint bucket tool on them, will make a large area white instead of just the grey part. 

I tried using the adjust selected color tool but it's very hit and miss and only worked the way I wanted a few times. A lot of my pieces are in colour so doing just a general brightness adjustment skewers the colouring. Is there a tool that I'm missing that could make this easier?