Discoloration of gray blocks in castle sets?

I just got back a bunch of stuff from my parents that's been in storage since I was younger. One of the items was a large tub filled with LEGO
blocks I had as a kid, mostly castle stuff. I found peeron.com where you can see the instructions to these, and was able to recreate several great old castle sets:
6012-1 Siege Cart 6074-1 Black Falcon's Fortress 6016-1 Knight's Arsenal 6049-1 Viking Voyager 6066-1 Camouflaged Outpost 6039-1 Twin Arm Launcher 6018-1 Battle Dragon 6042-1 Dungeon Hunters 6081-1 King's Mountain Fortress 6061-1 Siege Tower 6022-1 Horse Cart 6060-1 Knight's Challenge (incomplete, unfortunately)
In addition, I have the instructions to Black Monarch's Castle (and tried to put it together, but am missing a lot of pieces - mostly the curved tower tops and some of the black window toppings) as well as the 6080-1 King's Castle, and the 6077-2 Forsestmen's River Fortress. I'm uncertain if I have enough pieces to complete the King's Castle or River Fortress without cannibalizing some of the above sets I've already completed.
Anyway, I noticed that there are either 2 distinct colors of gray blocks, or else some of my blocks have "yellowed" over time. Both colors are the typical "light gray" but some are decidedly more of a yellow or tannish tint than others. I'm wondering if I've just managed to mix blocks from different sets so the gray is not exactly the same...? Or is this a known problem with some of the gray blocks changing color over time? It's weird because there is not a spectrum of color; there are 2 distinct colors, one more of a pure gray and one that's noticeably yellowish. Anyone else run across this? I'm thinking I may try to swap out pieces from the sets to get all the "yellowish" gray blocks together in a set if possible.
Since I can't complete the Black Monarch's Castle (but have a bunch of the figures and probably 85-90% of the needed blocks) I thought I might try to use the black pieces and change the King's Mountain Fortress into a "Black Monarch's Mountain Fortress". This might free up enough gray blocks to complete the King's Castle, which would be cool - giving me 1 castle for each of the 3 factions.
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