Why do Drosera leaves change color?

What you are seeing is the maturation phase of the plant.

As with the sunworts and Venus Flytraps of North America, sundews multiple by sending off "runners" that set root some distance away from the original plant. You also see this with orchids and what Americans know as "spider lily"'s.

I imagine the bud growth you are seeing at the center of the plant is the beginnings of a "runner".

The red in the young leaves of many plants is from their anthocyanin content. This might also apply to the hairs on your plant.

The anthocyanin seems to protect the tender young leaves from certain UV wavelengths, preventing them from burning As they develop, the red colouring is hidden by an outer layer containing chlorophyll, with which the plant can start making use of that UV radiation for photosynthesis.

As this process requires moisture, when leaves dry out (from drought or from seasonal changes in deciduous plants) to the point when photosynthesis cannot take place, the chlorophyll layer is lost and the underlying anthocyanins show through as the reds of autumn, or the browns of dessicated leaves.

I think your Audrey is just turning green from growing up. I am not aware of mosquitoes or flies being attracted to red, so this should not reduce the plant's chances of catching something to eat.

For the last 3 months, has Seymour fed it?

Just joking... Audrey looks healthy but may be going dormant. Sundews often become dormant for 3 to 4 months (usually during the winter).

The reddish color to the "sticky hairs" is often related to activity of insect capturing capability which is high in young plants and tends to taper off as the plant matures.

My sundews always did better in a topless aquarium. The side glass filters out the UV light and the partial enclosure usually keeps the humidity higher (and still allows insects to enter).

If repotting, use only peat (1 part) and Horticultural grade Perlite or Vermiculite (1 part). Never let the soil dry out. It is best to water with deionized water.

Did you ever give it an insect? This is the best fertilizer to use.

Some species require more light to remain active (and thus reddish). You might try shading most of the plant but allowing one edge to get some direct (through glass) sunlight.

Edited:

Most Drosera species contain Betalains and lack Anthocyanins as the red pigment. This is why additional light might bring back the red color. Just be careful not to expose the whole plant. just one edge to stronger lighting to test.