The lowest I ever used was around 4000k. The light did an okay job at growing my plants. But once I bumped it up to 6500k I noticed a growth difference, not to mention the plants just looked better under this spectrum.
Are you using the screw in variety? If so, it shouldn't be too hard to walk over to walmart and pick up some 6500k bulbs. In my opinion, anything under 4000k doesn't look good, and doesn't grow plants well. I don't know the exact science why that is, but it is what it is.
-John N.