A while back I posted a workaround for maintaining all the IRE values, even those above 100%, when using Colorista in Final Cut Pro. With Final Cut 6.0.2, this workaround is no longer needed (yay!), but you'd never know until someone told you. Allow me to explain.
Here's a shot in Final Cut 6.0.2. Note that the windows are above 100 IRE (click to enlarge).
If you apply Colorista, you'll see the values get clipped to 100%. This is not cool.
To fix this, you first need to select Sequence > Settings and set the sequence to always render in high-precision YUV:
When you go back to the timeline you'll see that the highlights are still clipped. But the key is in the wording—always render in high-precision YUV. So go ahead and hit Cmd + R to render the shot:
Your highlights are back! Final Cut is always using a lower-quality render setting as a placeholder until you actually render the shot. When you do, both Final Cut and Colorista do their thing in full floating-point color.
If you make a change to Colorista's settings, you'll get your red line back and your apparent clipping:
...but as soon as you render you'll get all your highlights back: