The colors relate to the name of the currency used in the country.
Green countries have “franc” in their names – could be Central African Franc or West African Franc or their own country name Franc e.g. Congolese Franc
Red has “shilling” in their names.
Yellow has “pound” in their names.
Orange has “dinar” in their names.
Blue has “dollars” in their names.
And Purple has “rupees” in their names.

The uncolored ones have very local names. Although Moroccan dirham has roots in Arabic and is shared with other countries like United Arab Emirates.