Magicky

Black or blue definitely! :)
Black is probably the most common and formal colour to use to write, though blue is equally nice. I'm. not sure about green, it's nice, but some people will not accept writing in that colour. Plus, light green can be difficult to see in. And I guess I just don't like writing in red - my own opinion. :)

if you are on mac, press command+4, then select the area that you want to screenshot, and as it says preferrably the entire screen. If you are on windows, press windows key+shift+s then capture the selected area, again, preferrably the entire screen. An alternative way on windows is the PrtScn key, which will automatically capture the entire screen. I will suppose everyone knows how to take a screenshot on a phone.
(for anyone who does not know how to take a screenshot)