Or it might be a matter of clearance after all. Grey is for everybody, including possible passengers and assorted other civilians; you need some sort of a clearance to use the express turbolifts marked in red, or enter the engineering spaces, or use the transporters, or visit the personnel residing in cabins thus marked; and only command staff is cleared to use the yellow doors (such as the one for the Captain's Locker Room in "Charlie X"
)...
Timo Saloniemi