There's a phone socket on the wall just outside the "networking
cupboard" at home (well, built-in wardrobe with the router & modem on
a shelf). I'd like to change it to a double --- phone beside RJ45 on
one faceplate & connect the RJ45 to the router inside. Instead of
wiring a socket on the wall to a socket inside the cupboard, then
connecting a patch cable to the router, is there any reason not to
just put an RJ45 plug on one end of a piece of cable & the RJ45 socket
on the other end?

If so, what is the colour-coding to use?

Thanks.