Don't worry I don't know either
Actually I have a small idea is just it takes so long and I can't find the exact value.
Dude, I literally just explained it, it took me 5 minutes to find the bytes that set the palette for Alex's pants.
But seriously, if you really want I don't mind telling you how I found them. You just need FCEUX, it's awesome. If you really want I can just change all the necessary bytes and send you a patch.
But I think helping you figure it out for the future would be more useful to you (give a man a fish etc).
So, step by step. I opened RCR in FCEUX and started the game, and Alex pops up. I pause and open the hex editor and look in the $200-300 range (the area in RAM known as sprite RAM). There you find 256 bytes dedicated to sprites: the NES can hold 64 sprites in memory, and each sprite is described with 4 bytes (any more than 64 on-screen and the game has to switch between them every other frame, which is why you see flickering). Remember, each sprite is only 8x8 (or 8x16, but only if you choose to do so for the game, which this one doesn't), so our Alex guy has several to make up his body. For more information on what the 4 bytes mean, read this:https://wiki.nesdev.com/w/index.php/PPU_OAM
To keep it short, the first and last bytes say where on the screen to draw the sprite - obviously this can change every frame with moving sprites, though not all sprites have to move. The second byte says which tile it is (open the PPU viewer to see what tiles are in memory at that moment). And the third tile sets attributes, namely whether to flip the sprite around (when the game begins Alex is horizontally flipped), whether the sprite should be behind or in front of the background, and crucially, which of 4 palettes to use. This is the smallest two bits, so if your byte has x0 to x3, you can see the palette. Alex's shirt uses the first palette (0) and his pants uses the second (1).
With all this in mind, I look in RAM for any sprites with 30 or 31 (the tiles for his pants). It's easier before any guys show up cause there aren't so many tiles to check.
Anyway, I see the first two sprites have exactly that, and their attributes are 41 (horizontal flip, palette 1). So I change it to 40 to change to palette 0, advance one frame, and hey presto, his pants change to white.
That's all great, except now I need to know the location in the ROM that TELLS the game that his pants should be palette 1. So I open the debugger and set a breakpoint for when the game writes to that attribute byte - in this case, $2E2 (and $2E6, but they're probably next to each other). I click Run, and find two instructions that write to it, but the key one is at $EEEB, because I can see where the number comes from. Directly above this instruction, it says it loads it from $8ED7, adds $40 if necessary (for the horizontal flip) and then writes it to the RAM.
So I go to $8ED7 in the hex editor and right-click, "go here in ROM file". That takes me to $CEE7. I can change the 01 to 00 and unpause the game - aha! One leg has permanently changed. Where's the other leg? Well, I try $CEE8 but that doesn't seem to do it, so I change $CEE6 instead. Woo, got it.
Well, that didn't take long, for Alex's pants, though obviously finding the location for every other character in the game will take some time (and don't forget this is only his pants while walking, we need to change his kicking, crouching etc), but it shouldn't be too much trouble, and once it's done, it's done.
I hope that was some help to you, and it shows how a little assembly learning can go a long way with your ROM hacking. Hell, I knew nothing until a couple of months ago.