Both lumens and Kelvin are important but for different reasons. The amount of brightness (lumens) is going to be more important though. Without adequate brightness, it really doesn't matter what the Kelvin (range of light, from yellow/amber to blue/purple). I published two articles on Light in January/February, 2014, and these articles may be of some help. The 6500K fluorescent tubes will produce more lumens than the 4500K tubes.
I am a little bit confused. You say you just purchased T5 fixtures but then say that you are using T8 fixtures. I prefer the T5 fixtures because you'll get more lumens from those. The Kelvin will be the same from either T8 or T5. 6500K is great for plant growth/greening. The 4500K stimulates flowering. If you are using the T8 fixtures, they should be adequate supplemental light for a single shelf. Just remember that you'll lose lumens very rapidly as the plants are positioned further away from the fixtures. Thus, you'll want the tubes as close as reasonably possible to your plants. I haven't used T8 fixtures in quite a few years, but I had my orchids within 6" of those tubes, probably more like 3" away. If you are using T5 fixtures, you'll be able to get more lumens at the plant-level than with the T8. I now use T5 HO fixtures and I have to position my orchids at least 24" from those fixtures. These fixtures produce huge amounts of lumens (30,000, 1" from the tubes!) and get hot as well. These fixtures that I now use are 4'x6tubes.
