Using the blue/yellow filter will boost color saturation, especially—you guessed it—blues and yellows.
Paul Hayes at Sky & Telescope has written a great tip about how to use the iPhone’s accessibility features to turn the iPhone’s entire display a specific color profile. For example, if you need to shade the iPhone’s entire display permanently reddish in order to preserve night-time dark adaption, you can do that. This technique would be particularly handy for amateur astronomers. While some astronomy apps have this feature, this tip applies to the iPhone’s display across the board. The tip is beautifully described, including an explanation of accessibility shortcuts, and also invites exploration for those who have certain kinds of color blindness. Check it out.