Thanks @Zoe7. I had a quick look at what I think is impasto gel which I'd like for the texture and brightness but I'm needing to thin the paints with flow medium as it is so thickening them might not work? Adding white is a good idea too. I also just had a quick read up on pigment stuff and think maybe I have used a couple that are transparent rather than opaque (didn't really ever consider the difference between those things!). I've got some learning to do!

Thanks again. It's good to know acrylics do that and that they dry duller - at least it's not just me then!