Ever dye wool tartan

[figheadair]

The idea that 18th century tartan was always dull is a fallacy. Those that could afford it would generally opt for the best and brightest shades.

With regard to your question, I think you mean/should be referring to, staining tartan rather than dyeing it. The former will tone down a pattern, the latter will alter it and depending on the colour and weight of dyestuff, obscure the design.

If you want to dull down a red based tartan you could steep the cloth in a bath of weak tea or coffe and that should dow the trick. Worth trying with a similar swatch before going the whole hog.