I'm always amazed at the range of paints available and how many different types modellers use. For me, it's always Humbrol enamels, and largely now the Luftwaffe-specific shades. I don't have a particular process to prepare them, it's just whatever takes my mood on the day. I use either proper turps or white spirit to thin them and there's no colour you can't make from them if you want to try your own mixes. I can spray them or use a paintbrush and still get equally good results.
I appreciate what some others say about how smoothly certain paints spray over others, but if I think the contents of a tin need a shake up, then in goes the thinner, stir the contents for a few moments, put the lid on and give it a damned good shake. Decant it from the tin into a glass paint jar and if it pours in a little too thick still, I just add more thinner in small amounts and stir everything with a soft, clean paint brush until I get the consistency I think will work. My general rule is that for spraying or brush work, paint should be about the thickness of milk. Oh, and always test on an old kit before you run the risk of ruining your current masterpiece... :-).
With the Humbrol Luftwaffe-specific shades, I'd say that the maritime 'black green' is a little dark, so I aways adjust it with some of the lighter dark green shade added in; the RLM 74 dark grey shade should have a bit of a greenish tint which doesn't always come through, so I add in a little dark green if needs be, and the pale blue undersides are usually a mix of pale blue, pale grey and white until I'm happy with the shade. Also, the varnishes can be a bit hit and miss, especially the gloss varnish which, while good and does the job, can remain tacky (sticky) to the touch for days! The matt varnish occasionally has a slight 'powered white' fleck through it in some areas which only becomes noticeable after it dries in my experience. So, now I always gloss varnish with Future floor polish, and when I'm ready to dull it down, I mix the Humbrol enamel gloss and matt varnishes together - erring more to matt - and when applied over the Future, a nice dull sheen is the end result, all of which can be adjusted to suit your own preferences with a bit of practice.
Some people have said the range is more 'gloopy' than the original Humbrol enamels of yesteryear, and I'd agree, but it just means I have to thin better and perhaps apply an additional coat. If brush painting, the first couple of thin coats can look dreadful, but persevere and the tone will correct itself with each application; indeed, the 'dreadful' look can be quite helpful if you like or intend your model to look very worn. Not every aircraft looked lovely and clean after a few sorties against the enemy, and we've all seen photos of shabby looking aircraft.
I also now venture into WW1 German aircraft kits, and I can still use all the WW2 Luftwaffe shades on these kits as well, so they're easy to use and actually, the main reason for me using this range is that I like the shades and most of them can be applied straight from the tin with little if any adjustments, so across my collection, the shades become standardised whereas before, dark green and black green could sometimes turn out more lime green and grass green... those were the days prior to experimenting, clearly! 🙂
I also elect to make my own blackwash which I still struggle with on occasion, but generally, I like the effect and it saves me money buying weathering colours/powders etc in a shop.
Remember, this is your hobby, it's not a race to complete a kit to someone else's expected standards. Take time to enjoy it and experiment and never be afraid to say you got it wrong... you'll always have the chance to improve the next time. Some examples below - sprayed, brush-painted and blackwashed... happy modelling :-).
7 attached images. Click to enlarge.