Will stay the same

Ce vert est parfait pour elle.

You missunderstood something in your sentence.
The word “vert”, the way you use it, is actually not an adjective, but a noun. By using the demonstrative adjetive before it, it becomes a noun. In french, this is called an adjectif substantivé.

I don’t know the rule for the gender, but i know that the noun “vert” is masculine.
Finally,

Cette verte

Does not exist.
But it can agree the plural case :

Ces verts sont parfaits pour elle.

Invariable colour adjectives

It is quite easy to know if you have to agree the colour adjective or not.
To agree with the noun, your colour adjective has to complete those 2 conditions :

• Not to be a derivation of a noun or any other word.
  Noir and rouge only exists as an adjective colour : they agree with a noun.
  But orange is also the fruit, marron is also a vegetable : they do not.

• To be single. bleu agrees with the noun, bleu profond and bleu cobalt do not.