I am using the sovereign states shapefile (ne_110m_admin_0_countries.zip) from http://www.naturalearthdata.com/ and joined the attributes with a spreadsheet file that codes many (but not all) country codes into 0,1,2, and 3. I would like to color each of these four codes with a different color using the Symbology->Unique Values tab, but the coloring is not correct. Furthermore, it seems to change randomly if I zoom in and out. The difference between the two screenshots is just the zoom level and you can see how the colors change.
I had this problem several times already after joining tables, but I could never figure out the source of the problem.