Colour picker to PICK ONE SPECIFIC COLOUR to SELECT ONLY!!

For example: only SELECT the grey pencil lines but DON'T SELECT the BLACK or RED PENCIL strokes nearby or on top!
That we we can ALTER those specific lines ONLY!! ( For example colour change them afterward, copy paste or change size or move them only)
Currently it's VERY TEDIOUS or IMPOSSIBLE to PICK SPECIFIC Lines when there are many..Would be GREAT for SKETCHING!! And that way we could SKETCH in ONE colour and draw in ANOTHER COLOUR on TOP and STILL be able to change the older lines below only.. Or delete ONLY them!