Trawling back through past editions of the Lincolnshire & Humberside Transport Review - the definitive publication that, thanks to Alan Tye (one of its founding editors), charted in exceptional detail the fleet movements for over two decades - provided no reference to a prototype livery trialled in late 1986/early 1987.
For this new colour scheme would be worn by the fleet in the same proportions as the outgoing caramel and cream, and before that blue and cream, but would comprise a fresh new orange. The particular shade of orange would be British Standard 5252, Colour Reference 08E55.
A little bit of detective work has revealed that the bus chosen to wear this new livery was likely to have been Fleetline 122 (OJV 122S). This is based on it being recorded that 122 was the first Fleetline to be outshopped in the eventual orange and white livery that was ultimately adopted.
Referred to by the paint manufacturer, Mason's Paints, as Grimsby-Cleethorpes Transport Orange 1987, the bus was duly treated and local managers and councillors were invited to visit its unveiling at the Humber Royal hotel in Grimsby one afternoon. As was befitting the time, a "lavish banquet" was laid on for the guests, which included managers and councillors in charge of Kingston upon Hull City Transport (KHCT), who were so impressed with the E-Type paint used, signed off a large order with the paint supplier to have their entire fleet painted in the product thereafter.
While GCT's managers and council leaders liked what they saw, and I believe initially signed off the colour scheme, it was later decided that the orange was too different to that already used in the company's publicity. While timetables at the time were printed on a beige paper, they did feature the stylised GCT logo and black and orange stripe. This was also worn by the entire fleet, painted on top of the caramel and cream livery.
A large amount of money was spent on bus stop stickers and more stylised fleet name vinyls, using the same shade of orange that the company has used since 1985 when BAM Associates designed the logo and introduced the use of orange as a colour.
The two shades of orange didn't compliment each other and rather than throw away all the publicity, it was decided to change the prototype livery instead. GCT asked for a livery based on the orange used in their publicity and stylised GCT fleetname. The mix would be referred to thereafter as WD40, though sadly as I discovered, the proportions of the constituent colours have been lost.
This new, bespoke shade of orange would be referred to as Grimsby-Cleethorpes Transport Orange 1990, giving the impression it was adopted some three years later, when in effect I believe the '1990' was added to give it a more modern feel. It was applied to the fleet from 1987 up to and until the Stagecoach purchase in 1993, when Stagecoach's then-corporate stripes livery would be applied to vehicles likely to remain in the fleet (though a couple of closed-top Fleetlines were so treated).