Raventutor 11:
The entire vehicle would be Olive or Khaki Drab inside and out. The engine block could be painted as "factory" Blue or Red. The breech block of the 25 pounder would not be Olive or Khaki drab, blackish gunmetal colour. If you are building a NW Europe Theatre vehicle you could paint it in the "Mickey Mouse" pattern of British and Commonwealth vehicles. There are numerous reference books available to obtain ideas from. Check around and you will locate ones that will assist you in your builds.
A few reference websites for you:
http://www.canadiansoldiers.com/
http://www.mapleleafup.org
Plenty of reference photographs for you on each of these.
Hope this helps with your build.
Cheers;
Gregory