How old is the JVC? Is it a "genuine JVC" like DR-MV1, DR-MV5, or DR-MV7? Those earlier units had issues with their video circuitry being noticeably brighter than most others. If you've grown used to that look, the Magnavox recordings can appear dramatically darker, possibly due to a mismatch between playback circuits.
If the JVC is a newer post-2006 model made by LG, it should roughly match the output of the Magnavox.
Try playing a Magnavox and a JVC recording in a PC, and also in the dvd players of a couple friends/relatives. Do they both look look normal and roughly similar? If so, everything is fine and your JVC simply doesn't decode the Magnavox dvds properly. Just don't use the JVC to play the Mag discs and you should be OK.
If you notice a huge difference in appearance between Magnavox and JVC recordings when played on PCs or other people's players/TVs, the unit that produces the poorer recordings is defective and needs to be repaired/replaced. If that turns out to be the Magnavox, and you bought it at WalMart, their return/exchange policy is extremely generous: you should bring it back to the store for exchange even if its six months old.