Choose your main color
Find a hue you like and then in HEX color picker try to pick a color from this area:
This is gonna be the main color of your website. Every other color is gonna be derived from this one. I chose blue because it’s my favorite color.
Now let’s make all the necessary shades of your main color. Switch the color format from HEX to HSL.
For the light colors the only thing you have to do is change the lightness. Start with 100 (white) and then make a shade with 95 lightness, then 80 for example etc.
Now for the dark colors do the same thing like with the light colors but with low lightness values. Just be careful and don’t use pure black. The contract between pure black and white is too big. Start with lightness = 10.
And voilà. Your monochromatic color palette is completed.