Are you deliberately trying to pretend you don't understand evolution? I get it you don't believe it, but after so many discussions you cannot possibly say you don't understand this basic part of evolution.
In the above example, say you had a white insect that had a mutation that made it green instead. If the insect lived in an environment that had a lot of green leaves, that one change would give it an advantage, even without all the leaf like markings, so that insect would bemore likely to survive and thus the trait will be passed down.
Then, another mutation gives a mix green/light green, again that trait gives an advantage, it gets passed down, as it makes it look even more like a leaf.
Each change that makes it look more like a leaf helps it to survive and makes it more likely to survive. Repeat, repeat, repeat, voila, an insect that looks like a leaf. It is not one change, but many changes, each change that makes it look more like a leaf is an advantage and gets passed to the next generation. Insects have very short lives, so genetic change happens very quickly.