See: https://people.uta.fi/~km56049/finnish/partreas.html Seems that in sentences that translate in English to "it is X," where X is an adjective, such adjectives are in the partitive.
I think the reason for using "soda pop" is that soda pop can only mean a fizzy sweet drink, regardless of what englush you speak. Lemonade however, is a fizzy drink in the UK but not in the US, in Canada they call all sweet fizzy drinks "pop", in some places all fizzy sweet drinks are "coke", in some they are all "soda", but in others "soda" is bubbly water...the regional variety for what to call this type of drink in English is huge. But soda pop is unambigous.
"Oranssi" and "musta" are the nominative (basic) forms of the words and "oranssia" and "mustaa" are these same words in partitive.
The partitive is used here (limonadi on oranssia, mämmi on mustaa) is because "limonadi" and "mämmi" are mass nouns, i.e. nouns that cannot be counted/nouns that can be divided.
A bit of a brutal example. If you have a dog (countable), and it loses a leg, you cannot say that the leg is a (one) dog too. But you can have several dogs, e.g. two dogs. With e.g. sugar (mass noun), you can't have "two sugars", just two spoonfuls of sugar for instance, but if you remove some sugar from a pile of sugar, both the pile and the bit you removed are still sugar.
See: https://people.uta.fi/~km56049/finnish/partreas.html Seems that in sentences that translate in English to "it is X," where X is an adjective, such adjectives are often in the partitive grammatical case.