According to the media compatability thread (
http://www.cdrlabs.com/phpBB/viewtopic.php?t=6005 ), Memorex discs manufactured by CMC do not burn well in the Lite-On LTR-48125W, maybe they don't read well in the drive either. I'm not sure if the Memorex Color discs are made by CMC, but if they are you may not be able to read them in the LTR-48125W. Upgrading the firmware may help, although I have reports of Memorex/CMC discs not burning (and maybe not reading) well with firmware version VS08. There is a line of firmware for this drive that starts with ZS, I believe ZS0A is one of the latest. You can try flashing to it, but nothing is guaranteed.
Try to find out who makes the Memorex Color discs that you have by using the Lite-On software tools or CDR Identifier.