Now, I know this post is ancient, but I stumbled upon it looking for a reason to see why my Sim's cat, (whom died of old age), was red. From what I gathered, you guys said it was random, or had to do with how much LTHP (Lifetime Happiness Points) you spent. Well, when my cat, Gingerbread Newman, died, I had bought the Bottomless Pet Bowl, Super Swank Pet Bed, and the Pet Hygienator. These were very fulfilling, and were worth a lot (Worth exactly 35,000 LTHP.), but resulted in low LTHP at death (A total of exactly 2,246 LTHP.). A few days later, after the son witnessed his mom's ghost around 6 AM, the opportunity to revive a ghost came up. NOW, I know this was directed to revive the mother, Sarah, but I instead revived the cat, Gingerbread. Upon leaving the facility with his red-ghost cat, I realized the RED cat gained two new traits which it didn't have when it died. These two traits were Aggressive and Noisy, both negative traits. When we think of anger, we think red. Therefore, I think that it DOESNT matter what you spent the LTHP on, it depends how much they have at time of death. Having a low amount= negative ghost color (red), and negative gain of two traits. (in this case, Aggressive and Noisy) I feel like running more tests, and I may later, but I really wanted to point out this experience incase anyone wanted to know.
~Flower (P.S. The cat died, and was revived with, exactly 2,246 LTHP.)