The coloring is for reference only. The 24th is one box. On the 24th you transition from one phase of the event deck to another. I think that is what it means. Not positive.
The relevant rules are on the bottom right of p.3:
In this step, execute one of the 3 headlines on the General Zhukov card and their text effects (See Appendix D on details of specific General Zhukov cards):
• On the 1st and 2nd day resolve the top headline on the card marked (1)
• On the 3rd, 4th and 5th day resolve the middle headline on the card marked (2)
• On the 6th and 7th day resolve the bottom headline on the card marked (3)
The colors on the top of the box (covering the dates) are consistent with these instructions. The color coding is to remind you to start looking at a different phase's headlines (when you get to a new solid-color box).