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I am developing a model of physical track construction in ants and I have a problem. On my model, I show obstacles for the ants: they are represented by red patches. I now implement pheromones (the ants leave a trace behind them that diffuses and evaporates) and I do not know how to lay them on the obstacles. Indeed, it would be necessary that the obstacle patch (of the red color) could turn green during the time that the pheromone is present and then return to red thereafter. So how do you assign two colors to a patch?