There has been a thing happening with tartan designers recently where that traditional practice hasn't been followed.

The first time I remember encountering it was with the tartan Flower Of Scotland.

It's basically Gunn in Ancient Colours with one narrow stripe moved. But AFAIK the Ancient colouring (pastel) was specified and required of the Flower Of Scotland tartan- in other words there could be no "Flower Of Scotland Modern".

The same thing happened with the California tartan. It's Muir in Ancient Colours with minor changes. AFAIK California has to be in Ancient colours.

Then there's Isle Of Skye! A lovely tartan. Genius, in my opinion. It has three different greens. Change any or all of those greens and the tartan doesn't work as it's designed to.

Tartans required specific shades of a colour is becoming more common especially with many of the American tartans and corporate tartans. IrnBru requires those specific shades of orange and blue. Ten different US Universities might wear red, each University's precise colour of red considered crucial and is maintained on all uniforms, apparel, insignia, signage, etc. Tartans designed for these Universities would be required to be each one's exact shade of red.