Each critter is born with one color and keeps it for life. What you paint is the background — the thing a predator sees them against. Critters that stand out get eaten more; ones that blend in live to breed.
Offspring inherit their parent's color, plus a tiny random mutation. So the only thing that shifts, generation by generation, is the mix of colors in the whole population — its allele frequencies. No individual ever adapts; the population does.
This is exactly the peppered moth of industrial Britain: soot darkened the tree bark, dark moths were suddenly the camouflaged ones, and within a few decades the population had shifted dark — because the pale moths were the ones the birds could see.
Turn mutation to zero and evolution stalls: selection can only sort the variation that mutation supplies. Kill the raw material and there is nothing left to select.
Something in the simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.