My experience has been that RX-6's are "darker sounding" than CC213's. And between Yamaha C6's and CC213's, that's a toss-up, depends on the particular instruments.
But, "dark vs. Bright" does not completely describe the sound, thinking you like "dark sounding" piano does not mean you will actually like the "darkest sounding" piano the best.
With acoustic musical instruments, you won't really know what sound you like until you actually hear it. Once you heard the sound you like, don't let "dark vs. bright" stand in your way -- you know what you have heard, you like what you like.
(Similarly, don't fret too much between new vs. old Schimmel 213 either -- if you've found the sound you like, you've found the sound you like.
Both RX-6 and CC213 are good pianos and either one will serve you well.
If you cannot decide yet, go hear the instruments some more, perhaps play the pianos yourself and see if you might prefer one piano's touch over the other's. IMO, RX-6 and CC213 do not sound alike and do not "feel" alike, spend more them with both, take your time to compare them, and I suspect your preference will become very clear before long.