They choose the Spanish dollar and were considering a different language. Why not pick green black and yellow instead of red, white and blue?
Maybe you’re thinking of the Grand Union Flag - the British union flag in the top left corner with red and white stripes. The US flag is descended from the red ensign. Anyway, they didn’t adopt the Spanish currency. And they didn’t consider a different language (because they didn’t speak one).
They weren’t aliens - they were defending their rights as Englishmen, not starting a brand new culture. Maybe you should be asking “why are they now so different?”
I might also point out that at the end of the 18th century, the bright, vivid colors we know today as “green” and “yellow” didn’t exist, since modern dyes hadn’t been invented yet. If you wanted some fabric to be “green” or “yellow”, you used vegetable dyes, like onion skin. And they wouldn’t give you a modern bright green or yellow–instead you’d get a sort of dusty Martha Stewart interior decorating green or yellow. Picture in your mind the dresses that those girls in Colonial Williamsburg are wearing. The green and yellow color tones are all sort of dusty “off-colors”, not deep hues.
And you could expect both colors to fade badly after only a couple of years, just sitting there in the drawer, and if you hung it in the sunlight all day every day, as one tends to do with flags, you could expect the colors to fade within a month or two.
Black also tended to turn “rusty”, with both age and sunlight.
“Red”, however, came from insect shells or madder, did not fade as badly, and was considered a permanent dye, as was “dark blue”, which came from indigo.
White, of course, stays white.
The name dollar was chosen for US currency because the Spanish dollar coin was the most common one in actual circulation in North America; British pound coins were seldom seen on this side of the poind.
The Gadsen flag (“Don’t Tread On Me”) was yellow though, wasn’t it? Or is that a modernized version?
More of a mustardy yellow than a bright yellow.
Yes, they had “yellow”, there were any number of plants that would give you a “yellow”…
…although it still wasn’t the bright “chrome yellow” that we think of today, as in these flags…
…but, anyway, the problem was that it would fade. If you’re picking colors for a national flag that’s going to be flown in direct sunlight from flagstaffs all over your new nation, you want colors that won’t fade.
Red and dark blue were selected as armed forces uniform colors all over the Western World for that reason, too.
This Gadsden Flag is more of a marigold-petal yellow-orange.