May is used to say what is possible, factual, or could be factual. For example, She may lose his sight The factory may close down. Might is used to indicate what is, contrary to fact, hypothetical or remotely possible. For example, If you leave now you might just catch the train, If I win the lottery, I might retire.

There was a time earlier in English's history where this was a meaningful difference, but it's a very subtle difference nowadays. I agree with Kim on his distinction. "Might" to me suggests that you personally don't know, while "may" is simply stating that the colors of these birds vary.

They're fairly interchangeable, but 'may' sounds right to describe ladybirds in a general sense, as in this example. You could use 'might' more naturally in a sentence where you're talking about some specific ladybirds you don't yet know the colour of... "The ladybirds (on my window) might be black or yellow"

The short answer is that it doesn't matter which one you use. Both express possibility, for a long(er) answer, check https://en.oxforddictionaries.com/usage/may-or-might .