Opening PDF files sometimes result in wrong colors because of the included ICC profiles (Output Intent Profile). A typical example is text that is set in solid black becoming a CMYK mixture.
Procedure
When opening such a file you can remove the attached ICC profile with Preflight:
Go to Pre-Press > Manage Preflight Profiles. This will open the Preflight Preferences.
Create a new Preflight Profile and give it a appropriate name.
Edit the new Preflight Profile and go to the Color section.
Select the Enable checks without restriction check box.
Add/double click both ICC: unwanted profile Text/lineart and ICC: unwanted profile Image profiles.
For both profiles, choose Remove ICC profile.
Click OK.
Go to Pre-Press > Preflight ...
Select your just created Profile and Preflight.
A message "Removed ICC profile tag from ICC based color space" will appear.
As a result the CMYK mixtures will again be solid black etc...
Instead of creating the ICC Profile you can also Import the attached profile.