Actually, I don't know of any bacteria that normally change color in response to temperature changes. I work with a number of microbiologists(people who study bacteria), and I'm a microbiologist myself, and none of us has heard of such bacteria. There are some bacteria that have been genetically altered to change color in different experimental situations,though, and that may be what you've heard of. Some of them are pretty cool.

Even though I took a microbiology class in college and taught a microbiology class in graduate school, neither I nor my friends have heard of bacteria changing color at different temperatures.

I can think of a couple potential answers to your question:

1) The bacteria have photosynthetic pigment, which is breaking apart as the bacteria are dying.
Organisms that absorb light in order to convert ("fix") carbon dioxide into sugar are known as photosynthesizers. Plants are a good example of photosynthesizers.

Photosynthesizers have pigments designed to absorb light and change it into chemical energy. These pigments have color; in fact, this is why leaves are green. In most cases, for land plants, photosynthetic pigments are green. Certain bacteria living in water or ice are photosynthesizers as well. In the case of bacteria, the pigments are not just green (as in blue-green cyanobacteria) but can be orangish (as in marine cyanobacteria) or even purple (as in purple bacteria). When these pigments are damaged or when the organism dies, the pigments change color. This is why tree leaves change color in fall - the photosynthetic pigments are slowly breaking apart as the leaves die.

If you had a solution or a petri plate with lots of healthy photosynthetic bacteria, their pigments might make the solution or plate look colored. If these bacteria were to die due to too high (or even too low) a temperature, the pigments would break apart and change color, and thus the bacteria would change color.

2) The solution in which the bacteria are growing has a dye that changes color as the bacteria grow. Certain chemical dyes change color at different pH. The pH of a solution is determined by how much acid or base it has in it - vinegar is a good example of a solution that has low pH, or is acidic, and bleach is an example of a solution that has high pH, or is basic. There are dyes that are one color in acidic solutions (let's say yellow) and another color in basic solutions (let's say blue). If you added such a dye to a solution with growing bacteria and then added ammonia, a base, the solution might start out basic, or blue. Over time, the bacteria growing in the solution will use up the ammonia. (Bacteria need nitrogen to make proteins, and ammonia is a good source of nitrogen for bacteria.) As the bacteria use up the ammonia, the pH will become less basic and the solution will change from blue to yellow. In this case, it's not the bacteria that are changing color, but the solution they are growing in. Many bacteria grow best at higher temperatures. It might be that as the solution you saw warmed up, the color changed because the bacteria were growing faster.
Certain bacteria produce acid as they grow (lactic acid, for example), which would also lower the pH of the solution they were growing in.

3) A food tag, meant to monitor the presence of harmful bacteria, changed color at different temperatures. There's been a lot of talk recently about protecting food quality during shipping. Harmful bacteria can grow and multiply on meat if it is kept above freezing for a certain amount of time. These harmful bacteria (Clostridium botulinum is one example, Salmonella species, Listeria monocytogenes and E. coli are others) are always present in small amounts, but can cause food poisoning if abundant. The higher the temperature the meat is kept at during transport and storage, the faster the bacteria will grow and the higher the chance of someone eating it and getting ill. Unfortunately, there is no way to know if the steak you are buying in the supermarket thawed while it was being shipped. It may look fresh, but could contain harmful amounts of bacteria in just a few days in your fridge.

Scientists have developed tags that can be attached to the outside of packaged foods that will change color once the temperature of the package rises above 32F, or freezing point. For example, a green tag may change color to yellow or red. The color change is irreversible, even if the package is re-frozen. The way one temperature tag works is that a tiny wire bends and changes position as it warms. Once the wire reaches a pre-determined temperature (say 32F), the tag is designed so that the movement of the wire tears a piece of green paper, revealing a piece of red paper underneath. When re-cooled, the wire returns to its prior position but the red paper remains visible, indicating that bacteria may have already started to grow. In this case, an object is changing color with a change in temperature in order to reveal the potential for harmful bacteria, but the bacteria themselves are not changing color. Other food tags use the consumption of oxygen by bacteria or the production of lactic acid by bacteria - instead o