Hi Jeppe,
If you just want to adjust the highlights, then you can do this without having to use a separate keyer to isolate the highlights from the rest of the image. The color grade / correction filters that are included in RED 5 have a feature called a PixelChooser that performs this task for you.
• Apply the desired color treatment filter to the clip (perhaps BCC Tritone)
• Set up the color treatment (probably just adjust the White Color value)
• Go to the Matte tab in the control panel
• Set the PixelChooser to View Chosen Pixels
• Set the Channel to Luma
• Tweak the settings (most likely the Black Level) until you are only affecting the highlights in the image
• Set the PixelChooser to On
• View the result – tweak as necessary
I hope this helps.
Peter.