Learn Finnish: Haluatko vaalentaa hiuksia tänään? - Do you want to lighten the hair today?

It is derived from the adjective vaalea (light or blonde). By dropping the final vowel and adding the transitivizing suffix -ntaa, it becomes an active verb meaning "to make something lighter" or "to bleach".

Does vaalentaa only apply to hair?

No, it can apply to anything you are making physically lighter in color, such as paint, teeth, or a photograph. The suffix -ntaa makes it a versatile verb for causing something to take on the state of the root adjective.

Why is hiuksia in the partitive plural instead of singular?

The singular word hius refers to just one single strand of hair. When talking about the hair on someone's head as a whole, Finnish uses the plural. It is in the partitive case here because lightening hair is typically treated as an open-ended action directed at an uncounted mass.

Could I use the accusative plural hiukset instead of hiuksia here?

Yes! If you ask Haluatko vaalentaa hiukset tänään?, using the accusative focuses on the complete, finished result of lightening the entire head of hair. The partitive hiuksia used here sounds a bit softer and focuses more on the process of doing some lightening.