Download google's picasa as it has a thing on it where you can saturate the colour so if there is any colour there it will make it brighter! It's betetr than inverting it as you can actually see if it's grey or coloured!
I didn't I'm just ever so clever, I just pretend to be thick and then people don't ask me to do stuff! I have used picasa for a while and when I was uploading my test pic earlier I thought about it as the saturation just enhances all of the colour so I thought it may come in handdy espcially when you don't know if the line is blue or grey!