What software did you use to create a monochrome red color palette?

Hi, what do you mean exactly?

If what you want is to look for the code of the color you are looking for, you have information about it in these two links:

Colors and HTML /articulos/colores-html.html

If you are looking for a specific color I recommend that you visit this other one https://color.hailpixel.com/

I hope it works for you, regards