At the moment, topside vandal NPC's all use the same color of spray paint, and the system encourages the removal of that spray paint from said vandal which then in one way or another gets added to the economy.
Because the then single colored paints saturate the market, it limits the other colors available to people to use through other systems (at least from a player perspective from what is readily available on shelves, markets, etc).
By changing up vandal spray paint colors, and making it random instead of a set color, I think it would solve this issue. Also would give me some break from seeing the same PVC dressed vandals rocking their favorite color every time the instance happens.
TL;DR Give the vandals some color choice to stop the saturation of their bad spraypaint choices into the market