There's been a recent thread on WD3.4 on the basics of painting with acrylics using a black undercoat. My contribution was my favourite recipe for yellow. This is a very common problem and I thought I'd post the details here too.
It's a two stage process (at least):
- Base coat - Vallejo Light Brown (929) which is actually an orange brown.
- Highlight - add Vallejo Light Yellow (949) to taste for as many layers you fancy.
Both are very highly pigmented paints which cover well and, when mixed, produce a very deep and vibrant yellow which is great for small areas on 15 mm figures. It really pops. In contrast, Vallejo Light Yellow (949) looks too stark being essentially a lemon yellow.