I do love to mix colours. It is my favorite thing to do. I am thankful that I have had a keen sense of colour. It is usually a challenging task as there are so many choices to make and determining what colour(s) is missing.
A colour is one thing on its own, and then there is the fact of how an acrylic colour changes when it dries. Then there is using the same colour on two sides of the icon (or canvas) and it looks like two different colours, in relationship to the other colours next to it.
Deciding on the base colours (and combinations of) for a piece often requires tweaking; too bright, not bright enough, too cool, too hot, a blue looks grey, or green, a colour looks dead next to the other base colours, etc. And as any legitimate artist will attest, white and black rarely factor in as these with make a colour muddy. I did have someone who said they had taken post secondary art courses for awhile, and advised me to use white and black to brighten or mute a colour! Ummm! No!
For example, I wasn't happy with a starting colour tonight which was an orangy red ( almost a Cadmium Red Light ). It looked too orange against the other Phthalo Green colour. Adding a darker Cadmium red corrected the orange but turned it too much to red and still not dark enough. So I added some Burnt Sienna to darken it and to add some red brown to it. It still was looking too red against the Phthalo Green so I added a bit of Cadmium Orange which did the trick. Now, how to describe the final colour... a coral/redish brown? Thankful that I don't have to give a precise name for it, it just works. But no white or black anywhere.
I will use some white to create lighter tint of a colour but never just white alone. I will always add another colour if I want to gradually work to cooler, warmer, pinker, bluer, as I go along. For darkening a shade of a colour, I will very rarely use black. Depending on the colour, I will use Burnt Umber for warm colours, Raw Umber for cool, but my all time favourite is Anthraquinone Blue. I generally avoid black in my iconography as it generally represents an absence of God's Light. What will seem black in my icons is anything from the Anthraquinone Blue, a dark Violet, a dark green or blue, a dark grey etc. They will vary depending the other colour(s) they are with. The same goes for white as highlights. Most often the highlights are very light pinks, blue, etc. but appear as white in relation to the other colours.
And there's the whole problem of mixing greys!! A tricky colour. Some of the challenges stem from the black being used as many commercial paints can have any number and ratios of colours mixed into them. It can make for surprises when you add it to colours or to white. Many will be variations of a warmer black (brown black) versus a blue black which is easier to control.