I've encountered this situation previously, I'm trying to apply a color change to a clip that has already been modified, in this case with a motion FX. I want to darken the clip, nothing complicated, but I'm not being allowed to do it, it won't apply. Can someone please tell me if I can't do this to a clip that has already had an effect applied to it?
Double click the clip to select it and then "Edit" - this will open Boris Red and you should be able to go from there. Make sure you don't slap your forehead too hard when you realize how easy it is ;-)
In Red, under the "Filters" menu you'll find "Color and Blurs". There's a number of color adjustment filters that you can experiment with. When you select one to try, a settings adjustment window will open and near the top you will see the word "current" inside a white bar. Click and hold on this to display a list of presets (most, but not not all filters have presets) that you can use as a starting point. Any custom adjustments you make can be saved and loaded for other clips. You can also apply multiple effects to a clip - as with After Effects, changing the order of the effects might also change the final result. One drawback though is you don't have the ability to use the Video Signal waveform/vectorscope function in Red like you do when you make adjustments in ColorFX. As well, in order to see the results in real time when you play the timeline you'll have to render. Under the "Tools" menu you can turn on "Vertical Render" to display the effect when you park the timeline player to check levels and such without rendering but I suggest turning it off again after you check - I've had problems working with it left "On" all the time, but maybe it's just my system. Another nice feature, when you apply an effect in Red, back in Media 100 you can "Apply Effects to Selected Clips" under the "Tools" menu (just as you can with ColorFX and Audio Effects)... a real time-saver.
I suspect once you experiment a bit you might find yourself using Red for more than just titles in the future... let me know how it goes.