I meant to bring this to GitHub but haven’t, my apologies. I find that following the color choice in Apple Calendar app (both Light and Dark) is an excellent guide. For example, when choosing blue as your calendar color, it has a light shade when coloring the block of time 9am-10am. It also has a darker shade of blue in a form of thicker vertical line, to show which calendar (Work, Personal) it represent. It is legible in Light mode and is even better in Dark mode.
mmm…
is this iOS?
On mac I have
PS
I don’t want us to abuse the forum; this thread isn’t about VisualDiffer 
Can you start a thread just for VisualDiffer?
I have PM you instead.