My preferred process (subject to change without notice) is to work up studies in acrylics and finals in oils. Acrylics dry fast, which is an advantage for tweaking colour or value by the “paint over ’em until it sings” method. Matching those choices for the oils is pretty straight forward, while the acrylics are not because they have that nasty little shift when they dry. Yeah, I care about consistent colour on my studies… once I’ve made up my mind that is.
One solution to keeping acrylics matchable is to keep the piles of paint wet. The trouble with this theory is that they dry fast (doh!) – especially when the thermometer is topping 40ºC (104ºF)…
So, how? I spritz the blobs of paint with water (or water mixed with a little retarder if I’m feeling flash but I don’t think it makes much difference) then cover it with cling wrap. I do this in such a way that the edge can be lifted just enough to slide the brush in. An occasional extra squirt helps, then a good freshen up with a little more water and new cling wrap at the end of the day. Perfect for a couple of weeks if you must… Toward the end of that time it may be getting a bit cruddy to use but it’s fine for getting a perfect match. I generally just keep it going until the pile runs low and then match it with a fresh lot. And on and on. Playing tag with paint can keep a colour going for months if I need to.