I've searched on many forums to find this answer and I'm in a bit of a pickle with it all.
I'm currently editing a short film and when I've come to do my primary color correction as normal i'd add the corrector to the clip, tried both adding it onto the timeline clip and dragging it into the filters tab in the viewer. But it doesn't apply only when I render.
Now I decided to apply a 3 way color corrector and that works a treat, no problems no need for rendering before i can view my correction. If needs be i'll just use this one but I prefer using the single colour corrector.
It's a project due friday and I'd rather get this sorted sooner rather than later...
Its a free download and may restore normality to your system. If it doesn't then I have to be honest it is not something that I have personally encountered so perhaps someone else has come across this and can explain your issue but pref trash would be my first port of call.
1. Whenever Final Cut starts acting weird, it is time to trash FCP preferences. Preference Manager is a good option, you can also do this manually, which I've written about here: http://www.larryjordan.biz/trouble
2. The single-wheel color corrector is OK, but designed for business graphics. Most of the time, you have more control using the 3-way Color Corrector filter. You get the same level of gray-scale control, but much more control over color adjustments.