Green and yellow light are very poorly absorbed by most plants (thus the prominence of green and yellow plant coloration; those colors are being reflected more than less represented ones). Red light is actually completely usable by most plants and is optimal for flowering. Red light alone is not that useful for this application, but important nonetheless. Peak light absorption for plants occurs at around 430 nm (quite blue) and 680 nm (quite red) wavelengths, which makes 6500K bulbs optimal as their phosphors typically generate light peaking fairly near those wavelengths (if you really care about it, you're best off analyzing the exact emission spectrum of the given bulb as the Kelvin rating scheme for bulbs is actually only loosely related to actual output spectra when dealing with emitters that are dissimilar from the black body radiators the scale is modeled on; LEDs and fluorescent tubes emit light in a very different manner than incandescent bulbs). Drifting upward toward 10000K will generally reduce bulb efficiency for plant growth purposes while compromising aesthetics (unless you like unnaturally blue-white washed out tanks); there is essentially no benefit to utilizing 10000K bulbs in terraria unless the particular bulb has atypically high output peaks at wavelengths corresponding to plant growth. The abundance of 3000K (primarily for flowering), 5000K, and 6500K grow lights are evidence of the above statements. Furthermore, the emission spectrum of the sun is approximately 5800K and one can probably safely assume most plants have developed to optimally utilize that light, with some exceptions.
In short, if you really want to maximize growth cheaply with a T5 fixture of that sort, you would likely be best off replacing both bulbs with reasonably good 6500K T5s, which can be had for somewhere around $5 each.
