I think the more common color is red. I believe some species of Aloe produce yellow flowers.

Planto

According the Definitive Guide for Aloes, Aloe vera only comes in yellow flowers and all red-flowering plants are actually one of several other very similar species (which would also explain why so many Aloe veras are thin, spotted and weedy looking while the 'true' plants are large, robust and pale in coloration without any spots).

Hmm... I was under the impression red was more common. You learn something new everyday, thanks for sharing. :)

Planto

Yellow is the only color I have encountered, and they are not very floriferous. Some people think there is only one Aloe, the Aloe vera, and some people confuse the hybrid (sometimes called Aloe vera) with the species.

The low growing soft plant with spots is the hybrid, sometimes called Chinese Aloe. This is probably the most commonly encountered Aloe in the world. The juice from the small soft leaves can be used to cool burns. The species described as Aloe vera is a larger plant with teeth that can rip you up. The clump in Baltimore at the Rawlings Conservatory covers an area about 3ft X 3ft and produces several flower stalks every year, but there are many more plants than there are flowers.

Here in southern California Aloe vera is one of the few species of Aloe the bloom every single month of the year... or at least one can always find one in bloom... the same plant will not bloom that often normally, but most individual plants, if put in enough sun, bloom at least 3-5 times in a year.