In many countries, having lighter skin is considered desirable because it indicates that you and your ancestors were not working outside very often, which suggests that you are part of a higher social class. Most white people, coming from more northern climates, do not turn brown under normal sun exposure, they turn red or pink. This is also what the term "redneck" refers to, that their necks and perhaps arms have turned red but the rest of their skin is still pale. As such, if one person refers to another person as a "redneck" then they are literally attacking them for the color of their skin, also likely because of what their social class is. Therefore, since liberals tend to insist that racism is often a mere reaction to skin color, they are by their own terms being racist when they use this term.
You state that as if it were a fact. Please provide your source. https://en.wikipedia.org/wiki/Redneck
Looks like you just provided a source for me. The Wikipedia article you've linked says that "redneck" is similar in meaning to the terms "cracker" and "white trash", both of which are recognized racist terms for white people.
You seem to be slipping a couple of predicates in with your conclusion there and both could do with a bit of evidential support. Do “liberals” (whatever that means today) generically insist that and do “liberals” generically use the term. If you could definitively identify individuals doing both of those things, you could level your accusation at them but tying it to an poorly defined label for a potentially massive range of people seems flawed (and maybe a little ironic in context).