My guess would be that this is in terms of reactivity and toxicity and the likelyhood that these factors will kill you. Yellow things like lead or lithium won’t necessarily kill you, but they will poison you pretty good. Red elements like potassium and sodium would potentially blow your tongue off due to their reactivity. At the least, they’d likely give you gnarly, severe chemical burns. Licking red stuff like mercury or arsenic will most likely kill you. Purple stuff is either extremely rare, unstable, and/or extremely radioactive. Licking it would cost a fortune in lost materials and likely straight up kill you from radiation poisoning or some sort of other reaction. For instance, francium, you’re definitely dead from radiation poisoning. So

“please reconsider” = you’re going to die painfully and horribly.

“you really shouldn’t” = it’s probably going to kill you, and, if you do survive, you probably wouldn’t want to be alive long after anyway.

“maybe not a good idea” = it might not kill you, but you’re gonna be pretty sick.