I have been breeding Rex for twelve years, and in those years, I have had Ambers and Reds together the whole time.
The easiest way to tell a red from an amber is to check the color of the kits in the first few days. Reds, even chocolate based reds, will appear fawn to orange (or rusty orange for a chocolate red) in the first few days after birth. An amber, on the other hand, will very closely resemble a chocolate otter, and only after a week or so will begin to move that really short hair further from the body, and thus begin developing that orange ring. For the next four to six weeks, these babies will continue to grow that original baby hair our to a very long orange band, as well as a pretty long undercolor band. The tipping at this age is VERY slight, as the tipping is relative to the coat size when the kit was born. a big hint that the animal is not red (or at least not black-based red) is a chocolate-tortish tint on the sides of the four to six week old kit.
As the baby grows from six weeks upward, the new coat will begin to form, coming out with a more distinct tipping and a redder band if it is an amber. If the baby is a red, it will simply molt into a sharper red color, there will not be black tipping. A smutty red at this age will have smut on the lower hindquarters, and very light red undercolor on those smutty areas.
Another strong hint on these guys is the points. A red will have the smut on the ears appearing to be ear lacing, but they VERY rarely will have smut on the tip of their tails. Ambers have a tail that is almost pure chocolate.
As they get older, around four to five months, ambers and reds are quite different. A red, when the fur is blown, will be a red that fades to a lighter red, with maybe a grayish tipping. An amber will be a chocolate tipped, with red that fades to blue undercolor.
Hope this helps, and good luck!
P.S. From the picture, I would say the animal is an amber. The chocolate rabbit next to it kind of makes the camera wash out the chocolate tipping. But based on the heavy chocolate tail, I would assume its amber. Its hard to make a guess unless you're looking directly at the animal and blowing into their fur.
