Learn Russian: Оба крыла белые.

The word for wing, крыло, is a neuter noun. The quantifier meaning 'both' has two forms in the nominative case: оба for masculine and neuter nouns, and обе for feminine nouns.

Why is крыла in the genitive singular?

When оба is in the nominative case (acting as the subject of the sentence), it behaves just like the number two (два). It requires the noun that follows it to take the genitive singular form.

The plural of крыло is the irregular крылья. Why don't we see that form here?

Because the rule for оба requires the genitive singular, we build the word from the singular stem, resulting in крыла. The irregular plural form крылья would only be used if the grammar called for a plural case (for example, if 'both' was in the dative case: обоим крыльям).

Since оба acts like the number two, shouldn't the adjective be in the genitive plural (белых)?

If the adjective was placed next to the noun to say 'both white wings', it would indeed take the genitive plural: оба белых крыла. However, in this sentence, белые is the predicate (Both wings are white). Predicate adjectives describing a plural subject simply take the nominative plural.