To make something opaque = opaquen?

There's opacify:

to cause to become opaque.

It sounds a bit awkward/made up, but the meaning is unambiguous.

Make transparent is more difficult. Clarify could work in certain limited contexts, but not in yours, I think. Unfortunately, I think a multi-word construction such as SetTransparency is your only choice here.

Opaquen is not a word. According to the Random House Dictionary, opaque is an acceptable word as an adjective, a noun (i.e. something that is opaque), or as a verb (i.e. to cause to be opaque).

However, people reading your code (undrestandably) might not realize that you are using opaque as a verb, so it probably is not the best option. I would reccommend SetOpacity.

I don't know of any verbs for your uses of to make something xxx. If there are any, they will likely not be known to your average audience, so it's best to stick with the to make form for the sake of clarity.

I've never seen opaquen used; in its place, I've simply seen opaque verbed. For the inverse operation I've seen deopaque.