Hawk Colour Schemes
I can't speak for RAF Valley but a series of conspicuity trials were conducted at RAF Chivenor during 1991 / 1992. The first Hawk, XX255, was resprayed in an all-over black scheme at St Athan in September 1991 and the trials were conducted between late September and early October against three other Hawk aircraft, XX325, XX331 and XX349.
A second stage conspicuity trial was conducted the following year to determine a highly visible paint scheme for the Hawk aircraft and during December 1992, at least four aircraft were noted in a variety of temporary schemes:
XX176 Matt Black overall.
XX179 Matt Dark Grey overall.
XX281 Gloss Black overall.
XX299 Matt Grey / Green camouflage (?)