A solo run, which is what it sounds like you may be trying to do, of Colosseum is a bad idea - it's much harder than the main game series games and involves all double battles. I'd suggest purifying and training some shadow Pokemon up to Umbreon's level.
In pokemon colosseum, at lv.36
pokemon do not evolve on pokemon colosseum
Get the umbreon from celadon mansion, and trade it over to ruby saphpire, emerald, diamond, pearl, and platinum and level it up at night. If it loves you it will evolve.
At level 37
Level it up with maxed out friendship at nightime.
In pokemon colosseum, at lv.36
you need to catch a shadow Pokemon, get the purple gage to turn to white by walking around with it, battling with it, calling its name when it goes into hyper mode, and giving scents obtained in Agrate Village. Then once it can be purified, go to Celebi's shrine in Agrate Village and press the "A" button while the shrine glows green, select the Pokemon ready to be purified, then watch. All EXP earned through battling (If it battled) will be given once it's purified. Then you have a regular Pokemon ready to be leveled up.
Get the umbreon from celadon mansion, and trade it over to ruby saphpire, emerald, diamond, pearl, and platinum and level it up at night. If it loves you it will evolve.
The final Colosseum is the Deep Colosseum after the Game, Pokemon here are Level 60-65, this is where you can catch the Pokemon you missed from the Admins earlier and a Shadow Shuckle.
pokemon do not evolve on pokemon colosseum
If you are trying to evolve it into Espeon or Umbreon, you have to LEVEL it up first!It takes a few hours/minutes (but mostly minutes) until Eevee has high happiness! You have to then level it up (either by battling or Rare Candy) by one level ...and then ...INSTANT ESPEON/UMBREON!Just to say, evolve your Eevee into a Umbreon because it is one of the ONLY Full Dark type Pokemon!
At level 37
no
Train your Pokemon and level them up by battling.
You level up by battling wild Pokemon, Trainers, and Gym Leaders.
Level it up at night
Collector Dean on Route 212 west of Pastoria City is a Collector with an Umbreon that is level 27.