Ok, I couldn't help but test it even though I'm not at work now... it works, and I feel a bit stupid, but thank you so much anyway!
For clarification for anyone reading this suffering the same level of daftness:
When using the "set material color" action, if you leave the 'material' field as 'none' then the action will only change that GameObject's color, however if you specify a material then the action will change the material and therefore anything in the whole scene with that material (even if you've specified a game object, I guess this gets overidden by the material setting if used).
I hope that makes sense and is correct, thanks again.