The yellow, light blue, and dark blue colors simply represent three different components of grammar in which we have grouped the lessons:
Parts of Speech
Parts of the Sentence
Mechanics and Usage
You can see a bit more as to which lessons fall under each category here:
https://www.grammarflip.com/grammarflip-lessons/
Of course, you can cover the lessons in whichever order you choose, or you can simply begin with Lesson 1 and begin working your way through each lesson chronologically or as the developmental level of your students may dictate.