By discoloration, do you mean those grey sooty patches on the stems? If so, that's most likely a combination of scarring from the mealybugs and sooty mold. Sap-sucking insects like mealybugs, scale, aphids, etc. excrete honeydew (basically sugar-filled poo goo) that provides a growing medium for a greyish/blackish mold. It can be pretty sticky and not easy to remove from plants. The mold itself isn't directly harmful to the plant unless it's so thick on the leaves that it blocks photosynthesis. If you find it on fruit, you can wash it off with warm water and a little soap and the fruit is still edible--but note that this applies to surface mold, not mold that gets inside the fruit and rots it.

EDITED TO ADD: If you mean some other color change, it could be a lot of different causes. Do you mean the leaves are turning yellow or bronze or developing spots? If so, could you post some pictures of more foliage instead of just stems?

Dealing with bugs like these is a catch-22. If you spray broad-spectrum insecticides, you also kill the predator bugs that eat them. And because these kinds of pests reproduce so quickly, you end up overwhelmed because there are no natural controls left anymore, so you have to spray more pesticides, and it becomes a vicious circle. But their overgrowth is often a sign of an ecological imbalance in the garden in the first place, or sometimes you just have really bad years for these sorts of pests. I would personally try spraying neem or insecticidal soap. They will still harm beneficials, but not as much as most commercial pesticides. The thing with pests like mealies, aphids, etc. is that they have a fast reproductive cycle, so whatever you use will have to be sprayed every few days for a while, since most pesticides don't kill the eggs. You can't expect one or two treatments to do the trick. It's an ongoing battle.