I believe that British Light Dragoon Trumpeters did wear reversed colours prior to 1812 but contemporary illustrations are hard to come by. It seems that the colours of the facings worn with the reverse colours were not always blue as one might expect.
In 1762, before the light cavalry adopted blue uniforms, the 15th Light Dragoons were illustrated in a series of paintings. They wore red with green facings and white lace. A trumpeter was illustrated wearing a green jacket with white facings and red lace.
In c1790, after the change to blue uniforms, a trumpeter of the 10th Light Dragoons was painted wearing a yellow jacket with red collar, and with lace instead of cords. From 1792, black bearskins were ordered for trumpeters, and the same painting shows a large hussar type busby with a peak and a yellow feather. I assume this headgear was reserved for full-dress.
c1800 two trumpeters of the 17th Light dragoons were illustrated wearing red jackets with white collars.
It seems that there was no strict regulations covering the dress of trumpeters, and the regimental Colonel had some freedom in choosing their uniform.
Regards
Gavin