Learn Spanish (Latin America): Mi colega es muy simpático y optimista. - My colleague is very nice and optimistic.

The word y (and) only changes to e when the word right after it starts with an "ee" sound (spelled with i- or hi-). Because optimista starts with an "o" sound, we just use the normal y.

Since colega ends in an "a", why is the adjective simpático masculine?

Some nouns in Spanish ending in -a can be either masculine or feminine depending on the person they refer to. Because the sentence uses simpático (which ends in -o), we know this specific colega is a man. If it were a female colleague, it would be Mi colega es muy simpática.

Wait, if the colleague is a man, shouldn't optimista be "optimisto"?

No, adjectives that end in -ista (like optimista, idealista, or pesimista) are the same for both masculine and feminine. There is no such word as "optimisto"!