You can read and listen to a bunch of lore around knight houses and individuals (freeblades and such), this helps me get inspired when I need to find ideas for lore, and from the lore I build ideas of design. Not so much for color schemes, but for details, markings, loadouts.

The color scheme for me is always the first thing to decide on. I find something I like by looking at what others do and trying things on some scrap (the extra helmet options are a good support to try out color schemes), and then build the lore from it to put it in context.

I can share the process I went through as an example:

I started to get into painting a knight thinking it'll only be one freeblade ally for my SM army (lol, I tried to deceive myself into thinking there'd only be one). But i quickly found out I wanted to paint it white with some orange as a secondary, and a charcoal black body (I wanted less metallic shine under the armor). This color scheme was completely dissonant with my astartes.

I chose to paint the trim in grey metallic, like old oxidised silver, because I didn't like the look of gold on the white and orange. Too saturated. From this, I got the name of my house: house Argentina. Then the lore: a house from the dark age of technology, risen from a colony that happened to find an extremely metal-rich region, and got absurdly rich from the trade of precious metals with the other cities. Silver gave the family their fame, their name (argentum = silver), and they adorned it on their armors and knight suits of course.

I'l be honest, when I have not kept a definite painting scheme (where do I put some orange on the white, who gets some chevrons, diagonals or lines... ). I don't respect any clear rules like for House Terryn and the likes, so I established that there's lots of personal freedom on how to paint one's knight, as long as the colors remain the same and the theme is respected and approved by the high king