I noticed that my gravatar has changed about 5 times for the last month. As for now, my gravatar on quant.stackexchange is brown, for the SE user it's blue but for the MSE it is red (while it was blue too 2 days ago). What causes a gravatar to change?
I'm pretty sure you didn't specify any e-mail address. In this case your gravatar is based on the last ip address used to access each site and will change if you access the sites from multiple locations. You can fix this by specifying a fake e-mail address in your profile if you don't want to put your real e-mail address. The appearance of your gravatar is then based on that address.