Size: An esrog’s minimum size is a kebeytza, which is approximately 56 grams. (ibid/29)
Green: An Esrog which has not yet ripened to the point that part of it has begun to turn yellow is Pasul unless one is sure that the Esrog will reach this stage. The custom however is not to take such an esrog even if certain, unless it has begun to turn yellow. [Ideally however the entire esrog should look yellow. It is better to take a yellow Esrog with Beltlach then a green one which does not have Bletlach.]
Black: A black colored Esrog is invalid. In those countries which generally grow green Esrogim this invalidation applies even if it has a slight black tint. However in those countries that grow black Esrogim, a light black Esrog is valid being that this is the normal way of growth in these areas and it is not considered an abnormality. However a very black Esrog which is similar in color to black colored people is invalid even if it grows in Africa.
How to cause an Esrog to turn green:
Placing apples around an Esrog helps it turn yellow.
 Sefer Haminhagim
 Hiskashrus 948 footnote 143
 Kashrus Four Species p. 17