Trying to get better color

[Quadracer041]

I have a question, why is it that sometimes the color in my photo's looks really good and other times it looks so flat or blah? These 2 pics where taken the in diffrent spots so obviously the sun was in a diffrent place but it was a tottaly clear day. The white balance was set to auto for both pics. Should I always be using a gray card? Would raising the exposer comp have helped for the dull pic? Looking to learn what I am doing wrong so I can correct it.

The color in the first pic looks much brighter then in the second, why?