According to my textbooks, early morning (dawn)sunlight is rated between 2000K and 2500K. So any bulb you can obtain in the 2000K to 3000K would be sufficient. For reasons of practicality, I would use the same color temperatures for the afternoon (dusk).
If you're lucky enough to have a 3 tube hood in which each light can be independently controlledy, I would light the tank as follows:
Let X=Early Morning Light Bulb
Let Y=High Noon Light Bulb
Let Z=Mid Morning Light Bulb
Early Morning: X
Mid Morning: XZ
High Noon: XYZ
Repeat in reverse for afternoon.
And durations should be obviously shorter. So "early morning" in your tank would be 30 mins, "Mid Morning" should be another 30 mins, and then 10 Hours of "High Noon". Then repeat in reverse for afternoon.
Gradual light changes are easier on fish, they don't stress as much as when the tank goes from dark to *BLAM* high noon. Which makes sense, cause if the sun did that to us in real life, we'd be a little freaked out too!
I hope this info helps.
-Viciente