If you want to look at the last couple decades, there isn't much history on the field with anybody. Washington as much as anybody, probably...
I think we have a good history with Oregon, with each side having their share of victories:
1967 Colorado 17 Oregon 13 at Eugene (with Bobby Anderson at quarterback for the Buffs)
1968 Colorado 28 Oregon 7 at Boulder
1978 Colorado 24 Oregon 7 at Boulder
1979 Oregon 33 Colorado 19 at Boulder
1984 Oregon 27 Colorado 20 at Eugene (the Ed Reinhardt game - a bond was formed between the two programs after Ed was seriously injured during the game)
1985 Colorado 21 Oregon 17 at Boulder (I was at this game in Boulder sitting next to the Oregon section - a key goal line stand won it for the Buffs at the end. Important game in Mac's turn around 7-5 season after going 1-10 the year before)
1986 Oregon 32 Colorado 30 at Eugene (tough loss for the buffs on their way to a 0-4 start, but later in the year Mac got his big upset of Nebraska)
1987 Oregon 10 Colorado 7 at Boulder
1995 Colorado 38 Oregon 6 (Cotton Bowl - as I remember the Ducks were mad at Neu for running up the score)
1998 Colorado 51 Oregon 43 (Aloha Bowl)
2001 Oregon 38 Colorado 16 (Fiesta Bowl - Ducks get revenge for previous two bowl losses, big disappointment for the Buffs in an otherwise great year)
If Oregon was in our division they'd be a great rival.