Red font sizes in Typetester

Typetester user Jason from Robbinsville asked:
Why are some font sizes in the typetester drop down size list in red and others not?
The highlighted values in font size drop-down match the ones in Photoshop.
Those values are also known as the Typographic scale. Mark Boulton wrote a series of short articles on the topic. Here’s a quote from Five simple steps to better typography – Part 4.
In the sixteenth century, European typographers developed a series of typeface sizes, a scale (the musical analogy is a good one – stick with me). As shown in the diagram they are sizes we’re are all familiar with. Six point through to Seventy-Two point type has remained pretty much intact for over four hundred years. In fact, they are the default font sizes in many applications (give or take a few.).
So, what’s so special about these sizes? Well, because this scale of sizes has been used for centuries, if set correctly, type set in this scale will appear more pleasing to the eye and therefore more legible.