Kolega is one of the masculine words that end with "a", other "spectacular examples" are Tata-dad and mężczyzna-man. Those nouns follow more-less feminine declination pattern, but have masculine/masculine personal adjectives, pronouns, and verb forms.
Yeah, also, like: poczucie humoru = sense of humour. But here it's 'mood'. Also 'nastrój' could fit this sentence.