Hi Wagyx,
I think just let it as '#'. Since this is an web based application,
hex color format is #000000.
https://www.w3schools.com/html/html_colors_hex.asp
For the simple-off-viewer I was sending just six hex digits in the
url. And your web page also takes it as six hex digits.
For consistency I went back to sending just the hex digits on the
url. My site javascript reads arguments from the url and format the
argument string and adds the '#' for input to the multi-off-viewer.
So '#' never has to be in a url string.
Roger