I did it for color. There's a rare vacation event that let's you remove all orange and white from your deck, then the legendary vacation event can remove the lowest color to get that 2 blue down to 1. Or you can use the dog to aim for the "remove a Color" battle event that let's you remove a color after battle.

I got it for numbers. Took me 3 vacation events: remove the lowest, remove "2", remove lowest and highest. Tried hunting for it specifically for quite a long time with no success, then got it after I gave up and started doing other stuff.
Some tips:
- the most useful pilot is Yongtong;
- the most useful talents are Vacation Refresh and Memories of the Past;
- common (grey) events don't remove energy, rare (blue) have 3 (including 2 that can remove multiple colors/numbers), legendary events have one.