I drove into a colleague's car at work with a forklift am i liable for any damage?

If your job involves operating a forklift and while doing your job this happened, then it's the employer who would be liable, not you.

Now a "Park at your own risk" sign does not necessarily immune someone from their responsibility. However in your case, the sign is there to draw attention that the area may be unsafe and by parking there you're assuming that risk. So in that line of thinking, I think your employer's liability can be disputed.

Note that I'm not a lawyer by any means and what I said above is just my opinion based on the facts you provided.