Use two brands of paint.
Lighten one with a bit of yellow, khaki or light green instead of white.
Simply lighten one more than the other unless you're painting a fresh dress uniform.
Add a shadow line on the pants along the edge of the jacket. To get the shadow line, you can use a very dark brown or even black oil very well thinned
If the figure in in a fatigue shirt, use a much lighter or darker belt to emphasize the difference. Also, if this is the case, the shirt will likely baloon out at the waist giving you ashdow line above the belt. Hit the uppr part of the pant with some highlights to differntiate them.
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