In full disclosure, I do not use colored text, instead I prefer to use the foreground and background colors to indicate things. I do use special characters in my description field, to alert me to certain conditions in that entry. I do not use common characters like . or $ though, as it can be difficult to remember what they stand for (what does the "$" indicate now?). I use emojis instead. If you decide to use Mmm's suggestion, I would recommend the use of an emoji as your color indicator, as that can further identify what that color is indicating.
For those readers who are not familiar with the use of emojis, emojis are special ASCII characters that look like mini icons. The most common one is the "smiley face", and emojis are most often used in text messages. But since they are ASCII characters (UTF-8 (Unicode) character set), they can be used anywhere that regular ASCII characters are used. They can be placed in Memento text fields, in Javascript code (since they are characters, they need to be enclosed in quotes), and anywhere else text is used. Emojis use the expanded UTF-8 character encoding, similar to the expanded character set of languages such as Hanzi (Chinese) or Cyrillic (Slavic). There is a standard list of emojis that you can find online. The good news is that they are a standard. The bad news is not all systems or applications implement that standard. For example the warning emoji on my phone is an orange triangle with an exclamation point, where on the PC it appears as a white triangle with an exclamation point (if you are running Memento on multiple platforms (PC, Android, iOS), be sure that the emoji looks like you want it to on the different platforms. Also, not all keyboards support emojis. On my Android phone, the keyboard I use doss not, but the standard Samsung keyboard does. I just temporarily switch keyboards if I want to specify a new emoji, and then copy and paste the emoji after that.