🔴 = Unicode Character U+1F534
🟡 = Unicode Character U+1F7E1
🟢 = Unicode Character U+1F7E2

In hexadecimal.

how to display the color light that?

OP solved it , closing!

thanks @akar-0
so it's the fill method right?
fill (6) string (size_t n, char c);

(6) fill constructor
Fills the string with n consecutive copies of character c.

where is the reference to this method, couldn't found it?

string(1, toupper(s[i]))?
string(, )

Noted, thanks

Please use a spoiler flag when you post code, so that users who didn´t solve the kata cannot see it; I put the flag for you this time. Also use markdown tags to format your code or it's not usable. Refer to the documentation about this and more: https://docs.codewars.com/training/troubleshooting/#post-discourse

You should read carefully the description to understand why your code doesn't work sometimes.