I just painted a red headed girl figure 1/20 scale last week. Here are the Model color acrylics mixes I used.
Cavalry brown as the shadow,
70% red leather with 30%sunny skintone for the base color
Straight orange color for first Highlight
straight Sunny skintone for final highlight.
This worked out really, really well for me. If you want a lighter red hair color then shift the above mix 70/30% as your shadows and orange with sunny skintone as your base ans sunny skintone as you highlights.
I hope this helps