OK seems like everybody uses Red Felt for under strings etc.
I'm thinking of using black or dark blue on this 1912 5'3" Kimball grand.
Has anybody tried other colors than red?
Will I regret it?

The felt color has no effect on the performance. Just do your homework well and use the correct felt types in the correct places.

However, there is a school of thought of "Let a Steinway be a Steinway, a Yamaha be a Yamaha, A Kimball be a Kimball, etc." Edit: If you subscribe to this school, them of course you will want to keep the original color.

Then, there are those who will make major modifications far exceeding merely changing the color of the plate felt.

You might. Most factories use red felt because it improves the tone of their pianos. Bluthner uses blue felt, and that's why their pianos sound so bloody awful.

Everyone uses red because that's just about all that you can get from the regular suppliers. If you know where to look, you can get green, blue, burgundy, black brown and even purple piano felt.

Do you really think color effects tone/sound?

Not carrying yellow anymore?

It's more fun for me if I don't answer this question.

Red is better if you play mostly romantic era repertoire, but I would use blue if you think you will play mostly Jazz. Green might be best if you are into folk, or New Age. If you like to drink a lot while you play go for the burgundy.

Sun of a beach!
My monkey says to use purple ]....

This one was obviously restrung using blues felt.

I have tried other colors of felt and plate.

No regrets.

Your only worry is picking a color that might put a customer off.
Small thing are sometimes weighed far beyond their meaning.
Just like in plates, I would say that black is right out.
It shows every mote of dust, and is impossible to keep clean.
In addition, though I can't say about modern dyes, historicly, black dyes have been very corrosive. I wouldn't trust them in contact with the wire.

Green felt always sounds the best. Why don't they use it for hammers?

I am so glad that my antique Bluthner has red felt. I always did wonder what its secret was!