For flowering plants, id use one 6500k bulb and one 2300k bulb in your fixtures.

I think marigold and zinnias will be considered long day plants for flowering - I am not sure though. The usual technique for such plants will be to grow them under cool white for 12/12 hrs to make them grow first and then switch to warm white for a longer period say 14/10 or even 16/8 to induce flowering. Why not try and post your results. It will be interesting.

Thanks everyone for your comments. I just learned that cosmos and zinnias are facultative short day plants which means less light during the day can actually cause them to flower earlier than they normally would.