Gender and Colour
Why do some colours like green (vert/verte) change with gender, while colours like red (rouge) do not?
If the word already ends with ‘e’, then it already has the normal feature of a feminine ‘agreement’ for adjectives such as colour, like the example you give: rouge
This would not just apply to colours, but to any adjective ending in ‘e’ in the masculine form.
The same kind of ‘agreement’ exception also goes for plurality. E.g. ‘mon fils’ (my son) becomes ‘mes fils’ (my sons) in the plural form, because the normal addition of ‘s’ or ‘-es’ is not required since the singular ‘fils’ already ends with the ‘s’ sound and spelling.
I hope this is relevant and/or helpful