Based off this Reddit, it is the following:
Level | Color
100 | Green
200 | Teal
300 | Purple
400 | Red
500 | Yellow
Repeating over and over. You get additional gems on your portrait and
your portrait shape improves each cycle.
To give you an idea, this is what the borders look like:
According to the image source:
Your portrait badge art will become more intricate for every 25 levels you earn. Upon reaching level 100, your visible level will reset to 1 and you’ll don a badge that’s even more epic so that you can show off your dedication anywhere your portrait is visible.
I think the Wiki's description is better however:
- Player progression badge art will receive more decoration every 25 player levels.
- Every 100 player levels, the level displayed in the progression badge will reset, and the badge will upgrade with even more ornate
- The Basic Portrait Border will update every 100 levels with new art that matches the player’s progression badge.