Missions that you already have in your Journal are marked with different (lighter) background color in your Daily missions list.
No, it wasn't.
But, I think I took this mission before my account was moved to an other account.
With the migation, I lost lot of stuffs: my ranger rites, all my special bags (tokens, wigs etc), all titles won, and all stuffs in my appartment.
Therefore, maybe that's why it looked exctacly the same way ? (I guess it's not "common" to move a toon from an account to an other one?)
Any way, even if the color is different, as we don't know the colors signification...