Hi there, welcome back, you disappeared after a short while last time, but you posted some great stuff.
Great question. It is not Mars, although Mars is certainly the reddest major body in the solar system aside from Io possibly. However Io is more of a yellow rather than red (see my avatar, pretty well true colour) & Mars is a deep orange - red generally.
Amalthea as you correctly say is extremely red & is a great contender. Sulphur from Io coats Amalthea & the extra redness is due to the radiation envorinment & lower temperatures. Also Amalthea's neighbours, Metis, Adrastea & Thebe should also be very red.
Sedna also appears extremely red as does the Centaur object 5145 Pholus. I really do not know for sure.
I will investigate.