This is the 6258th most frequent Polish word.
białej
"Białej" is the feminine singular genitive or locative form of "biała," meaning "white."
In this sentence, 'białej' is used as an adjective (feminine singular genitive) describing the color of the dress ('color of white').
Maria kupiła sukienkę w kolorze białej.
Maria bought a dress in the color of white.
Here, 'białej' is an adjective (feminine singular genitive) describing 'sheet of paper' ('white sheet of paper').
Widzę białej kartki na stole.
I see a white sheet of paper on the table.
In this case, 'białej' functions as an adjective (feminine singular genitive), modifying 'gloves' ('white gloves').