Subtle and interesting shading can be created using inks. Unlike washes, they tend to more uniformly tint regions, rather than heavily pigmenting recesses at the expense of raised areas. Experimentation is recommended to achieve the results desired, the effort will prove worthwhile.
The more small details you can find to accentuate, the more realistic your forces will be. Try painting the canvas where the guns enter the turrets. Don’t forget aerial recognition symbols.