I use Todoist basic. Six of my projects have gray squares next to them, while two have red squares. How do you change the square colors, and what do they indicate?
these colors can be changed using the gear icon menu. If you hove over each project, on it's right side, you'll see a gear icon. Click on it and select the color you like from the colored boxes at the bottom of the drop-down menu.
These colors don't have any special meaning, you can use them to indicate a certain project group or just change the color of each project so it can be quickly identified on various search results as often the project name from which a task originates is being displayed on the right side using the color you've set for the project.