That's opal. Often what I see in rex is that the opals tend to have a very pale undercolor as juniors that later molts into more correct of ring color. If it is from lynx lines, though, it might not get the correct dark undercolor as lynx rex have been bred to have a lighter undercolor.
In Rex, lynx is a little bit complicated. Evans software can't deal with Lynx rex. Lynx rex (at least, most the lines of lynx rex that are bred to fit the lynx standard) are often genetically fawns, not lynx. The standard for "lynx" rex was written to describe a fawn; the fact that it shares a name and close appearance to "lynx" in other breeds (which is what Evans software would be calling lynx) is unfortunate. So in rex you have two unique genotypes & phenotypes for lynx: phenotypic lynx which always has a light undercolor, is born pink and is genetically fawn, and genotypic (true) lynx which sometimes has a dark undercolor, sometimes not, is born dark (lilac) and is genetically lynx.
In this case, what I would guess is that you ended up mixing a phenotypic lynx (fawn) with a true lynx (the sire is probably the true lynx, if he has a lot of Amber in his lines, as true lynx is a dilute of amber). The most expected outcome of that crossing would be opal, but you could also get lynx (true lynx and/or phenotypic lynx) if the parents carry chocolate and/or non-extension (respectively)
Lynx is one of the more difficult Rex varieties to start out with; not only because of problems like this, but also because it can be difficult to find good stock to bring in. Good Lynx rex exist but are much harder to find than a good rabbit of a more common color.