I am currently painting a figure which has yellow trousers. While I can mix beige with the base color for highlight I am not sure what color I should use for the shadows. What would be the best color choice? I was thinking about mixing in a little bit of brown, but I am not sure. Any suggestions?

Figure guys jump in - My first thought is to try a complementary color across the color wheel from yellow as a shadow color. So the color across from yellow is blue. Now be carefull how much you add, blue is powerful. A touch of blue may do it.
This is from theory not experience.

Here is one reference
http://www.mauigateway.com/~donjusko/othercolorwheel.htm

I almost always use Humbrol and I do it like this:
24 Yellow + 62 Natural Leather adding more and more Natural Leather and 160 German Red Brown for the darker shadows. Some times a little bit black for the darkest shadows.

Highlight by mixing white and flesh to the base. Another trick is not to use yellow for the basecolour but darken it and use straight yellow as a first highlight.

Toke, this is sort of what I thought.

A warmer yellow might warrant reddish brown while a cooler one could go with a green added in for the shadows. You might want to use what you consider your base yellow as the shadow color then go progressively lighter with the addition of white.