T5s are a lot hotter than T8s. Your plants probably burned from the increased heat. Hang the T5 fixture a few inches higher than you would the t8s.

It also depends on the difference in the spectrum of light that the two bulbs use. Softer lights put out more red light, whereas brighter lights put out more blue. When your plant produces the chemicals to absorb that light (chloryphyll), it produces more of whichever chemical's light is most plentiful.

In other words, your plant may have been conditioned to receiving more red spectrum light and thus when you switched the light you may have switched to a brighter, more blue spectrum light. Since the plant didn't produce alot of blue-absorbing chloryphyll, it had to start up that chemical process, which probably caused the color to go away. I had this same problem when moving my plants between lights in the spring.

The best thing you could do is to use BOTH lights for awhile and provide your plant with more full spectrum of light. Or if you simply prefer to use the T5, you could use both and then slowly wean the plant off the T8's spectrum by using it intermittently.

Hope this helps.

The red spectrum/blue spectrum has nothing to do with the size/efficiency of the fluorescent fixture and at that, it is an overrated/emphasized aspect of growing. The Kelvin temperature rating of the bulb only tells you the overall appearance of the light's color to the human eye but tells you nothing about the wavelength profile. Most fluorescents have a strong profile in all growing wavelengths.

Another myth is that T5s are exceptionally hot. I have hibiscus within an inch of my T5HO bulbs and there is no burning.