Sources mentioned below are documented on page 1. Click to enlarge images.
Beamish’s history of the KGL (Vol I page 81) states categorically that “the uniform of the … engineers was in every respect similar to that of the same corps in the British service,” though how absolute was this similarity is open to question.
For the pre-1813 period, the prescribed coat for British engineer officers was dark blue faced with black velvet, with regularly spaced buttons and unlaced, as in this description in the Buckmaster tailor’s book at the National Army Museum:
Dark Blue Coat Lind with White. White Turnbacks sewed down. Black Velvet Lappells, Cuffs, and Collar. 9 holes in Lappell, 4 in Cuff, and 4 in Flap, all regular. Buttons to stand under. Waistcoat Back 2 holes. Small Buttons at ends of Collar. 1 Gold Epaulette. No Skirt Ornaments.
The only evidence, to my knowledge, of the dress of a KGL Engineer officer for this period is in the Meyer ledger (thanks to Meyer & Mortimer and to Ben Townsend for access) in an account for Captain Müller; unfortunately, the military contents are limited, but we know that in 1810 the captain ordered a black cassimere waistcoat and four pairs of plain blue stocking or cloth pantaloons.
In 1813 the uniform coat of the Royal Engineers was changed to scarlet with mazarine or “garter” blue velvet facings and gold metal, as in this description from the copy of the Hawkes tailor’s book at the NAM:
Scarlet super fine Cloth Coat, strap lapel, 10 holes regular. 4 holes & buttons on round flap. 4 buttons on Cuffs, no holes. 2 holes ach side of back. 1 button no hole on Collar and 2 buttons middle of plait. body lin’d rattinett. white Cassimere Collar[?] lining, skirt lining & turnbacks. Mazarine blue velvet lapel facing, Cuffs & Collar. Uniform buttons.
Charles Hamilton Smith’s chart and plate show the British Engineer’s blue facings as a rich mid-blue shade. Though there is no indication of lace in the Hawkes description, Hamilton Smith’s plate shows gold lace on collar, cuffs and lapels; the latter are of conventional cut, rather than in “strap” style as in Hawkes (like the artillery jackets here). Smith shows grey trousers or overalls with a scarlet stripe.
Significantly, Hamilton Smith’s chart entry for the KGL Engineers for this period duplicates the British corps exactly. A plate in Beamish also shows a KGL engineer officer for this later period. However, there are troublesome anomalies here: his unlaced coat has no buttons on collar or cuffs, the breast buttons are clearly spaced in pairs, and the facings are of a much darker shade of blue, virtually blue-black, while the later version in Schwertfeger does seem to show black; this leads Mike Chappell, in his second Osprey KGL volume, to interpret the facings as properly black. (The absence of clear coat skirts in this image also leads Chappell to call this a jacket.) Beamish shows the trousers as plain grey.
At a later date, Knötel (Vol III) compromises awkwardly, by keeping the paired buttons but opting for mid-blue facings, while cleverly ducking the coat/jacket issue. All in all, the Beamish plate raises more questions than answers, and until further information comes along, we’ll just have to live with the uncertainty.