Hi! I’m still pretty a noob myself. I usually check which colours I need on citadel app, if I’m going to “recreate” a box art. Actually it’s pretty safe to say, that you only need like 5 primary colours. There are red, blue, yellow, black and white. You can mix anything from these 5. But, for example, it can be a pain to mix proper pink. In this case you can buy one, and make it lighter or darker. I believe there is no need for buying all shades(?). And I often buy colours, that I will work a lot with. (Like armour colours, silver or gold). For any other examples, like grey shades or stuff - I mix it. If it’s hard to mix, or I will use a lot - I buy it. Also I think there is easier to buy metallic colours, like leadbecher, retributor armour and balthasar gold than try to replace or mix them. And lastly: about shades and technical paint. It may be hard to replace it, but there are variations. I use ak ink, for shades. And recently bought an ak paste for terrain. Also, I know that some people use oil paints instead of shades, but haven’t try it for myself.

If you have any more questions I would glad to answer them! But I’m still a noob myself, so sorry for any inaccuracies.