Range: grey out numbers on tap or completion

[LukidayNL]

When playing Range with extremely large custom maps, I end up losing track of which numbers are already completed. Being able to grey out numbers by tapping on them makes it easier to keep an overview. This could be implemented similarly to the numbers in magnets. Tapping them once greys them out, tapping again brings it back to the usual color. An optional addition could be that the number greys out by itself once it is surrounded on four sides by blocks.