How can I tell healthy mycelium from contamination?

Healthy mushroom mycelium is bright white, ropey or fluffy, and grows out evenly with a fresh, mushroom-like smell. Warning signs are any colour other than white (green, black, grey, pink, or yellow), a slimy or wet look, sharp lines between growth zones, or a sour or musty smell. When something looks or smells off, isolate it and compare against a clean plate.