I have a lot of colours but I rarely use them all. I’ve forgotten why I bought most of them. The main ones are Titanium white, Ultramarine blue, Mars black (I used to use Panes Gray but Mars black is warmer), Yellow Ochre, Burnt Umber, Hansa Yellow, Quinacridone Crimson and Dioxazine Purple. Dioxazine and Quinacridone are pigments that were not used in oil colours (until recently). They are new (turn of the 20th century new). Dioxazine is particularly welcome because purple is difficult to mix. In my attempts to stay away from dangerous elements I have also used Naphthol crimson, which I like.
I’ve tried modern colours in an attempt to get away from Cadmium. My father had Cadmium poisoning (quite nasty) during World War II from repairing military aircraft. It’s a component in the dope applied to wing fabric. I’m sure there are lots of other nasty chemicals in our many other artists colours but ignorance is bliss.
My chief problem with acrylics is transparency. Sometimes it works great but at other times it’s a royal pain. Sometimes I put down a heavy coat of Titanium white then apply the colour on top. This usually works. The alternative is multiple coats of the transparent colour.
If I have too many colours I have difficultly determining exactly which ones to use. Sometimes a colour is difficult and requires a different pigment to keep the result from becoming muddy. It isn’t always necessary to exactly match a colour. I often try playing with the contrast rather than the colour and it usually works. Expectations also play a part. Water is often gray and muddy but I prefer it blue, so I use ultramarine and sometimes darken it with Mars black. I like the resulting blue water even though it isn’t accurate.