You can directly plot your countries visited map using an URL similar to shown below:
Here, after the #, you have to put ISO2 country codes separated by commas (with no spaces in between).
However, if you want to create multi-color grouping, then you can follow the patten used in HTML source code of this page.
Note that you can even annotate on this map! To do this, click on the download icon on right hand side of the map and then choose Annotate.
You can save the map with annotation.
This page should work fine in any web browser (desktop as well as mobile devices) - better viewed in landscape mode in mobile screen.
This map uses Eckert 6 projection, which represents countries according to their true sizes, unlike Mercator's projection where Greenland appears bigger than India!
However, this is a political representation of the world. If you want to view physical representations, there here are few maps for you.
Koppen climate map
World map by relative population sizes
World map by life expectancy