The first thing you must know is that in Color Stack Mode you do not set the background color for each individual card, you use the "color stack" array. Your original question suggests that you were not aware of this.
First, you need to define your "color stack" array. Right now, you have this:
MODE SCREEN_CS, CS_GREEN, GREEN, GREEN,GREEN
Which means that it is all GREEN (actually, you should use STACK_GREEN for the stack array; CS_GREEN is for Color Stack foreground colors). For grey, you need to do something like this:
MODE SCREEN_CS, STACK_GREEN, STACK_GREY, STACK_GREEN, STACK_GREY
Notice that I alternate between green and grey. You could use more colours, but since you only specified grey and you are already using green, this is the easiest way to use the color stack. The reason is that you can only advance the array in sequence, so if you are in the first colour (green), and advance it to the second colour (grey), then advancing it once more will bring it back to green in the third colour, and so on.
Then, if you want a BACKTAB tile to have a grey background, just set its "advance" flag by adding 0x2000 to its card value, or use the constant CS_ADVANCE. For example:
CONST MUR = $0800+18*8 + CS_ADVANCE
Notice that, as I explained in my previous post, that this will cause all BACKTAB tiles after this one to continue using grey as the background colour. Since you only want it to be in one card, then you must set the "advance" flag on the next card as well. What this will do is move the color stack array pointer to the third colour, which happens to be green again, like the first. This is why we made it green, grey, green, grey; so that you can just advance twice to get one card grey.
If you have multiple cards using the MUR constant consecutively, then you only add the CS_ADVANCE constant to the first one you want green again.
Does this make sense at all?