Any tutorial for editing textures should be helpful, whether it's for map or model doesn't matter, it's the same format either way: 8BPP BMP with dimensions that are multiples of 16 up to 512x512.
The
wiki and
The303's guide should be plenty helpful for that.
For the actual recolouring you'll have to export the texture you want to edit using a model viewer like
Solokiller's Half-Life Asset Manager and opening each weapon's viewmodel. Then you have to change the colour mode in your editor to RGB or similar (in GIMP it's Image -> Mode -> RGB) and then you're able to modify the colours freely without being limited to the palette. You can either use a tool to shift the hues (such as GIMP's Hue-Saturation) or paint the colour on a separate layer above with a colour/hue/etc blend mode. There are many ways one could go about this and looking up guides in a search engine for "<editor> recoloring" or something like that can be helpful if you've never done that before.
Afterwards just follow any guide to convert back to 8BPP BMP and use HLAM again to import the texture back to each viewmodel.