i dont know how to change the color, but i definetly can say you how to get rid of the whole thing
1) To turn off the logo: stsadm -o setformsserviceproperty -pn AllowBranding -pv false
2) To turn on the logo: stsadm -o setformsserviceproperty -pn AllowBranding -pv true
these commands need to be run on the server