Adding event layers

Hello

To paint a picture about my Plan in luxcal:
"I have 12 hours divided into 3 Categories of 3 hours each, and everytime a Category of 3 hours is finished I have a "buffer" or blank time-space of 1 hour, so I have 3x3 hours pluss 3 hours buffers spread over 12 hours."

e.g. Category Blue Color (3 + 1 hour buffer) it is relatively large time space. I need to add a "layer" of event as a _________ (fill in) of Blue Category.

What is my Purpouse?
I want to use __________ to separate 1 hour from Blue Category and make it e.g. Red and dedicate it for my worktime.