The officer was straight-forward enough, especially after his mounted counterpart. his buttons, hat trim, and sword hilt are silver, since that is the regimental metal of the Marines.
He still has a gold tassel on the sword, and the tip of his scabbard is brass. This may change as my research into these uniforms continues.
The drummer's uniform was the focus of many hours of obsessive deliberation. I found sources scarce on the subject, and its coloration the topic of hot debate. I ended up settling with a red coat with white facings and lace, and white small clothes, despite the contrary Army regulations.
The tipping point for me was a filming costume used in the Hornblower series. As the episode was set prior to the Marine's elevation to Royal status, they were still a white-faced regiment (1790s). Following the chain of my painting logic one step further, the series was costumed by John Mollo, a notable source on period uniforms.