The hands are a part of the weapon viewmodel, so to change them you'd have to remodel and texture new hands for each individual weapon model. You can use MilkShape to decompile the models and recompile them after editing, but obviously you'd have to learn how to create models. Alternatively, if you want an easier solution, you can try replacing the Half-Life weapon viewmodels with the ones from Blue-Shift, and use Jed's Half-Life Model Viewer to replace the textures for Barney's sleeves to make them a different color.