These color differences have been discussed ad nauseum on the forum in part because they are so subjective and elicit tons of opinions. It has been said that unless there is a truly striking difference the catalog publishers have done a disservice by listing these and creating more issues to try and obtain.
Here are three stamps from the same Kelleher auction presumably scanned on the same equipment:
Scott 63 Blue
Scott 63b Dark Blue
Scott 63-E13f Blue
All three of these stamps have PF certs.