There are two sets of tasks: 1 set of “linear tasks” that players work through until they are all done, and then a second set of “daily tasks” that randomly pop up once the first set is done. There isn’t a published list of the linear tasks as far as I know, but you’d think that what you’ve done would get you there (I would have guessed maybe get all colors to 10 or 20). It’s been over a year since I went through them myself, so I don’t remember that one at all. You might need to put a ticket in to support to find out if there is a bug and the task isn’t clearing when it should, or if you just haven’t reached the threshold necessary to clear it. Maybe it is 25 in all 6 colors, which you couldn’t reach until level 150? It only counts the base bonuses and nothing from your guild, hero class, etc. When you level up, it should show your base mastery on the left hand side when you are choosing which colour.