"Asubuhi njema kaka"
Translation:Good morning, brother
They change because of the difference between morning and afternoon. In Swahili, each noun belongs to a different noun class and the noun class it belongs to decides the prefix of the adjective describing it. The words "asubuhi" and "mchana" belong to different noun classes and thus, the adjectives take different prefixes.
In East African culture, you would use "habari za asubuhi" much more frequently than "asubuhi njema," especially as a greeting. Perhaps you see someone on the street in the morning, the first thing you would say to them is "habari za asubuhi." "Asubuhi njema" would be used more frequently when leaving a conversation: "Bye. Have a good morning!" I have never heard someone start a conversation or greeting with "asubuhi njema."