Hi Matt, well Winsor & Newton artists' oils and Grumbacher Pre-Tested are both decent quality so either is fine, although I'm sure where you are Grumbacher will be cheaper. With cheap colours like earths an economy range like Winton is often perfectly acceptable but you'll probably need to soak out excess oil from them if you want to maintain matt drying.
With regard to the two mars colours, Mars Brown was discontinued by W&N in the last revamp of the range because it can be mixed by the artist (Gold Ochre, Venetian Red, Mars Black).
You already have Gold Ochre, this should be very similar to the average Mars Yellow, W&N discontinued it for this very reason. Technically they are just two names for the same colour!
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Fleshtones are complex but they're not hard to mix and the key is simplicity. The fewer colours you use for each basic shade the easier it is to achieve consistent results. The paints you list amount to two yellows, an orange and a red; when you put it in those terms it shows it's a bit redundant
the simplest flesh mixes are based on a yellow, a red and blue (or black);
all fleshtones are within the triangle formed by these three hues, plus white of course.
If you don't use black or blue as a neutraliser one needs to use earths of some kind, as many modellers do, instead of true yellows/reds in order to keep the mix from being too high in chroma (too 'bright'). Using earths, particularly the umbers but any true brown, often results in mixes looking 'muddy' unless you're very careful, for this reason most portrait painters avoid using them except in their darks. And incidentally, you can't substitute an orange (like Burnt Sienna) for the yellow and red because you need to vary their proportions.
If you have Cerulean Blue here is the basic palette I would advise: Titanium White, Yellow Ochre (or Gold Ochre), Cadmium Red Light (medium is okay if you don't have the light) and a touch of Cerulean Blue. Variations of these colours will give you everything from a light highlight through to a medium shadow colour without the addition of anything else.
The basic light highlight colour is mostly Titanium White, some Yellow Ochre and a touch of Cadmium Red Light.
For a basic midtone, the same amount of Yellow Ochre, a touch less Titanium White, a little more Cadmium Red Light and dot of Cerulean Blue. An intermediary can be made by simply mixing this with the highlight, or you just blend between them if you like to work wet-on-wet.
I mix my medium shadow colours by blending the Cerulean with Yellow Ochre to the darkest green they'll make, and then mixing in Cad Red Light until it's a dull brown. This colour won't make muddy mixes if it gets into your midtone, as you might if it included Burnt Umber for example.
For the darkest shadows I begin with the basic shadow mix and add in a variety of other colours, but for simplicity you can limit yourself to a little Burnt Umber and black (I use Mars Black preferably because it's most opaque but any black will do).
Einion