I journal in black and red. Almost everything is black because I reserve red for anything on theme or particularly important to me. By “on theme”, I mean consistent with my yearly theme, which is to Make Art with Friends.
This has advantages in review and while journalling the day.
I am using a Recurring Journal this year, which means that when I journal I am seeing a past day on a different part of the page. If that past day was particularly red, I can try to match it. I can also easily see the red ink as a flip around. At the end of the year, when I review the journal, it will be easy to pick out the important parts.
During the day, I can see how much red ink I have or have not used and try to see if I can get something in to make the day more red.
Not every day needs to have some red on it, but sometimes I have a free moment, and since I have a low bar for making art, I can easily do it.