We all know that Kai is colourblind, and that Grif doesn’t make a huge deal out of her colourblindness. Probably because he’s so used to it and he doesn’t really see it as making his sister any different.
But imagine them growing up together and her being sad and upset because some asshoes were teasing her because her clothes colours don’t match right, and Kai slowly replacing the colours in her wardrobe with black and white and grey.
And Grif sees this and it pisses him off. He finds those kids and deals with them because no-one messes with his lil sister. Then he takes her shopping and he buy’s her colourful clothes, and sorts them by colour for her and tell’s her what will go best together. Imagine Grif describing different colours for her, using emotions and feelings and concepts instead of telling her that ‘the sky is blue’ like everyone else does. He creates these big long descriptions of things and her deciding that she likes the way Orange sounds best.
So when Grif goes away to the army he chooses orange armour to remind himself of what he’s fighting to protect.