I believe a mat pose or a material file will replace the shader nodes rather than add or change them .  Theres a bunch of (well several) python scripts to change things in the shader tree without blowing it all away.

http://www.sharecg.com/v/23657/Script/Set-diffuse-gray-script-,-for-light-set

theres one that sets all materials diffuse color the same . Edit the diffuse value for whatever color you want.