I'm not using Geneva colours as I'm not in the US, but I think I'd still be struggling even with them. Failing badly with mixing, particularly skin tones. I'm posting my painting, and pictures of my palettes.
Although I enjoyed painting and do somewhat like the result, the colours are a mile from accurate (and I need to make some adjustments to the eyes). No matter what I seemed to do, each time I mixed for skin I ended up on the same kind of makeup-like, "cheap foundation" colour. The palettes hint at this, I think. I couldn't get the warmth without going to orange or mud.
Could this be my base colours? I'm using a crimson rather than a more pure red. Time to switch to the recommended Winsor & Newton Geneva alternatives?
For reference, I shot the painting with a grey card in the shot, then white balanced and cropped in Photoshop. The palettes are just from a phone, so not balanced properly.