Having done a quick bit of background reading, just to confirm what has already been said, the following locos carried Regional Railways livery in 1994, which is pretty much when the livery was at it's zenith:
31410 "Granada Telethon"
31421 "Wigan Pier"
31439 "North Yorkshire Moors Railway"
31455 "Our Eli"
31465
37414 "Cathays C&W Works 1846 - 1993"
37418 "East Lancashire Railway"
37420 "The Scottish Hosteller" (A fairly late addition - Repainted during 1994)
37421 "The Kingsman"
37422 "Robert F. Fairlie Locomotive Engineer"
37425 "Sir Robert McAlpine/Concrete Bob"
37427 "Highland Enterprise"
37429 "Eisteddford Genedlaethol"
And in addition to D6975s comments, in 1994 the following pools had the following locos officially allocated to them:
Crewe Diesel Class 31/4 (North West Passenger)
31410 Regional Railways
31421 Regional Railways
31432 BR Blue
31439 Regional Railways
31444 Civil Link
31455 Regional Railways
31465 Regional Railways
Crewe Diesel Class 37/4 (North West Passenger)
37402 BR Mainline
37407 BR Mainline
37408 BR Large logo
37414 Regional Railways
37418 Regional Railways
37421 Regional Railways
37422 Regional Railways
37425 Regional Railways
37429 Regional Railways
Motherwell Class 37/4 (Scotrail)
For the Highland Mainline, Aberdeen to Inverness, Kyle line trains, etc.
37427 Regional Railways
37428 Trainload Petroleum
37430 BR Mainline
37431 BR Mainline
Motherwell Class 37/4 (West Highland)
Used on the Fort William sleeper, for Intercity, and West Highland freight amongst other things.
37401 Railfreight Distribution
37403 BR Green
37404 Trainload Freight
37406 Railfreight Distribution
37409 Trainload Freight
37410 Transrail
37423 Railfreight Distribution
37424 BR Mainline
Of course, these are only the official allocations, in theory, on paper. In reality, a far wider range of locos could be reasonably expected to "drop onto" any given Regional Railways passenger working: Certainly, a far wider range of class 31s were fair game on the Regional Railways workings in the North West, wearing BR blue, Civil Link or BR Mainline liveries at the time; the North Wales Coast saw a wider range of class 37s; and Motherwell had a sizeable fleet of "no heat" 37/0s and 37/5s that worked the Inverness and Aberdeen sleepers (in pairs, with a generator van) and got turned out for passenger workings on the Highland Main line and on the routes out of Inverness during the summer. In the latter case, many of these locos were the former boiler-fitted locos that had been the mainstay of the West and North Highland routes before they were ousted by the introduction of 37/4s in 1986/87.
And this was when regional loco-hauled passenger services were winding down! If you look back further to the start of the nineties, the range of locos used (and to an extent, liveries displayed before Regional Railways began to be applied) only increases!