Thats great Im loving the beard. Im guessing you are doing this for the PC version? Using a modelling software and texturing in PS?
Wargamer's been handling the majority of the armor tweaks and recolors, while I've been the one doing the tweaks for the face textures. I use GIMP, which is also freeware. GIMP can't work with the filetypes Space Marine uses natively like paint.net can, but you can download a plugin that lets it do so, and I have more experience with GIMP.
As to the 3d stuff, although its possible to do direct editing and importing of the 3d models, so far this has all been accomplished simply via clever and surprisingly easy shenanigans with file directories, 'tricking' the game into using, say, the Corvus armor parts from multiplayer instead of the default Ultramarines suit.