Both protectorate and colony are actually a part of a larger country, or they are both territories of a larger country. A protectorate is a state on its own which is merely being protected by the larger country, while a colony is an actual part of a larger country that is governed and ruled by that same country.
A protectorate state has the help of a larger nation when it is conducting foreign relations. While in the case of a colony, all foreign relations that it is involved in are created and actually dealt with by the larger country. Protectorate states are basically seen as alliances.
The more the number of allies, the stronger the mother country becomes. Colonies, in contrast, are usually viewed as threats to other nations, as it is actually the spreading of one country. Barbados was a protectorate state, which was ruled by Britain while Nigeria was a colony, ruled by Britain.