Question on how Evolution works to produce new characteristics

If at the time of the invasion of plants they lack any members with orange pigment whatsoever, probably not. It might develop during the takeover but it is very unlikely.The frogs might instead be dominated by the quickest variety of frogs that quickly dodge predators, rather than those that sit still and hide. Or the frogs that hide on the underside of leaves might win out, or the ones that burrow in the ground or swim in the water. Orange skin pigment isn't the only trait at work here; in the same sense that none of the frogs have the ability to shoot fire out of their rears and fly through the air as the perfect defense, a new threat isn't necessarily going to wipe them out if they don't develop such a trait.

And that is where you go wrong. Evolution isn't concerned with what they are better off being, it doesn't work that way. If the frogs get selected for the nasty or toxic ones surviving longer, they might develop the survival method of being garish and poisonous. In that case camouflage might actually be selected against, even though theoretically if they were all camouflaged they would have a higher rate of survival.The process doesn't run through what is most effective in theory, it needs to work with what it has.