I haven't touched teamcolour texturing myself yet, but this should be helpful:
Click me
Basically, the desired geoset of your model has to feature two layers: 1 - teamcolour; 2 - texture.
If you add the desired areas of your texture file to its alpha-channel (read
this tutorial to see how it's done), the model will feature teamcolour at those spots.
Since you posted this in the Requests forum, I assume you want this to be done for you, aye?
Edit:
I have edited the model and texture respectively.
I hope this is what you wanted.
Apparently, it's not as easy as it looks. Checking Arthas's model shows that you can not simply select the desired area and alpha it - it would be completely invisible.
You have to work around this.
What I did was:
Mark the desired area, add it to the alpha channel.
Duplicate the selection of the texture, desaturate it and set brightness/conrast to a satisfying level.
Copypaste it on the alpha channel.
That way, you will keep the structure of the selected area..