Yellow: Central Powers.
In World War I, there were no axis and allies like in World War 2. There was a block called the Triple Entente, that was originally conformed by Great Britian, France and Russia. Later as the war continued many countries around the world like the US and Brazil joined it. The other block were called the central powers, which was formed originally by the German, Austro-Hungarian and Ottoman Empire. As the war continued Bulgaria also join the Central powers.
