K.M.
I'm a photographer doing bluebonnet sessions, and I say color! Not white, but we can always work with white even if you do want to go with that. Last year I had my daughter in a really cute brown dress with pinks, oranges, and greens popping out and the color looked really cute with the blue flowers and green grass. I also did her in a white dress with little blue and green flowers all over it. I did a little girl in a bright orange dress and it was really cute. This year I'm doing my daughter in a yellow dress. Really, color is nice - and yes, don't mix patterns - if one child has a pattern, put the other in a solid that brings out the colors in the pattern.
About white - it's actually really pretty in the bluebonnets, especially dresses and such. Good luck! These are the sessions I look forward to the most each year! :)