Color fill eating up borders

[jjaaxx44]

Hello :)
I'm having issue where after repeated tapping on same area, fill will start eating up black borders.
tolorance is 10, I have created a demo, where on each touch random color will fill tapped area.

is there any way to reduce this?

original image

after few taps

after ~10 taps

[jjaaxx44]

Anyone?