Unfortunately you cannot apply Conditional Formatting to Listboxes.

TechHelp: List Box with Format and Color?

Not really looking for 'it cannot be done' but more like ' list box won't work but you might try''
Any useful comments?

You asked a specific question, and got a specific answer. If you watch the video Alex linked, you would see some ideas that you could maybe use.

If you want to use conditional formatting you'll have to make continuous sub forms for every one of those list boxes, and completely remake how the calendar works with VBA. Then you could color code fields inside the subform. Although at that point it's not even the same lesson, you'll be rebuilding the whole things from scratch almost. Some of the same theory would apply.

Here's some videos to get you started
Intro to VBA
SUBFORMS
VALUE FROM A FORM