For the white tail, you will want the Dominant White gene. It turns all black into white, but leaves the red/brown colors alone.
The wild-type color of a Brown Leghorn, plus Dominant White, makes the color called Red Pyle in Old English Game Bantams. Cackle Hatchery has some photos of them:
https://www.cacklehatchery.com/product/red-pyle-old-english-game-bantam/
If you look through the photos, you will see that some of them have more white and less red/gold/brown coloring, and some have less white and more of the other color.
You could selectively breed for the ones that have the amount of white and the amount of brown that you want.
To get the Dominant White gene, maybe just cross in a White Leghorn. For example, if you cross a Welsummer rooster with a White Leghorn hen, all chicks will look white, probably with a few black dots (because Dominant White is a bit leaky when a chicken has just one copy of the gene.)
Taking a daughter from that and breeding to a Welsummer rooster again, the chicks should divide into four color groups:
--white with leakage of black and maybe red/brown
--black, maybe with leakage of red/brown
--approximately Welsummer-colored
--like Welsummer coloring, but with white instead of the black
Take one of the ones that is like a Welsummer but with white, and breed to some more Welsummers, and you should get a 50/50 mix of birds with Welsummer color and birds that have the Welsummer-with-white coloring. Continuing to breed them with Welsummers will keep giving the same results.
You can eventually interbreed the ones that have Dominant White, and get some offspring to breed true for the trait (no longer produce some chicks that show black.)
If you start with a flock of pure Welsummers, it would be easy to add one or a few White Leghorn hens, and for quite a few generations you can have a flock with pure Welsummers and with some Leghorn-mixes that have Dominant White. As you see which ones are best for your purposes, you may decide to keep having both types, or you may switch over to having just one type or just the other type (pure Welsummers vs. Welsummers with a bit of White Leghorn mixed in.)
I notice from some of your other threads that you are interested in Welsummers. They would probably be a good breed to start with, since I think they have most of the traits you want. You may even be able to start with pure Welsummers and have exactly what you want other than the white tail. Mixing in a little bit of White Leghorn can get you the white tail and may increase the flightiness and foraging by a little bit.