] Here's an 1811 textile swatches page from the March issue of Ackermann's Repository, just to demonstrate that vivid colours were not a problem!

I really wonder how people can forget that military uniforms of the time were a rainbow of very bright colours. Dyes, at least, were available all right, from plants and with metal ground, too. Bright-blue indigo, and bright-red alizarin (forgot the plant itself, alizarin was got from the roots, gosh, beginning with "m"...). The fashion, of course, is another matter, not always brought about by availability, but your argument seems definite to me.