Patient (.iplan6) and plaque (.iplq6) files created by Plaque Simulator are actually Unix packages (folders disguised as files) that contain many embedded files including pictures of the plaques, CT, ultrasound and fundus images, patient IDs, custom icons and so on. The embedded images are saved in .png file format.
With the introduction of MacOS 10.13 (High Sierra), Apple changed the way that .png file gamma metadata is interpreted compared to earlier versions of the OS to be more consistent with other OSes. When PS6 (and other MacOS apps) open .png files exported by versions of MacOS prior to High Sierra, these images may appear "washed out" or excessively bright. The fix is to remove the now obsolete gamma metadata from these embedded .png files before opening the PS6 packages.
If you have updated to MacOS 10.13 and are experiencing this issue, please download the ImageOptim application, expand and install the app, launch it, configure its preferences as illustrated below, and then drag and drop any affected patient (.iplan6) or plaque (.iplq6) files onto the ImageOptim.app window. The gamma metadata will be removed from all the embedded .png images and picture brightness should appear as intended when the file is subsequently reopened in PS6.