Learn Croatian: Kakvu boju trebate, svijetlu ili tamnu? - What kind of color do you need, light or dark?

Kakvu comes from kakav, which asks "what kind of" or asks about the qualities of something. This makes sense here because the options are "light or dark" (qualities). If we used koju (from koji, meaning "which"), we would be asking the person to pick a specific, exact item from a known set, like pointing to three specific cans of paint and asking "Which color do you need?"

Why do we say kakvu boju instead of the dictionary forms kakva boja?

Because of the verb trebati (to need). When you need something, that thing is the direct object and must be in the accusative case. For feminine nouns ending in -a like boja, the accusative ending is -u, making it boju. The question word kakva must match it, becoming kakvu.

Why do the adjectives svijetlu and tamnu also end in -u?

They are describing the color (boju). In Croatian, adjectives must match the noun they describe in gender, number, and case. Since boju is in the feminine accusative case, the adjectives svijetla (light) and tamna (dark) must also take the feminine accusative ending -u.

Is trebate used here to address one person or multiple people?