Okay, I went and did some reading and I realize now that my earlier suggestions ("why don't they add a gene that causes 2 different coloured eyes or pink pads instead") make no sense at all. I understand now that the glowing cells can be tagged and tracked by scientists because they can see them and it allows them to better understand the course of diseases. So, I retract my somewhat silly suggestions but I still feel that perhaps the animals of science could be treated with a gentler approach. If the glowing cats were just being held by someone on that news footage while the lights were being pointed at them and cameras flashing like crazy etc.. just something or someone to comfort them it wouldn't seem so cold. But that is just the softie in me and I suppose sometimes I need to have a more logical viewpoint. Maybe they get lots of lovin' behind closed doors. I will certainly pray that they do.
Anyway, here are a few paragraphs that I copied from a site written by someone in support of the testing in response to someone who was not:

QUOTE FROM SOMEONE ON ANOTHER SITE:
"Under normal light say from an incandescent light bulb or in normal sunlight, the animals will not be emitting radiation like a Christmas tree. The fluorescent colors are only visible (yes, how could have logic guessed it) under a fluorescent bulb. Particular genes, extracted from other animal species which are naturally fluorescent (say from the ocean) are inserted into an animal cell. That cell will produce the corresponding protein which when exposed to a fluorescent light source will have its electrons excited so that the color will appear. Typical colors used for such tagging experiments are green (green fluorescent protein-GFP; perhaps you might want to spend 30-seconds of research), red, blue or occasionally yellow.

It is done to tag cells. To understand how an organism, especially a mammalian one as complex as a cat develops cell by cell, tracing the mitotic pathway, it is an excellent tool to be able to trace what cell ends up where. In fact Nobel prizes have been won for doing exactly that in the Nematode (C. Elegans) model, tracing all the cells from its original stem source. It would take textbooks to explain all the different experiments that can be done by tagging cells"