What do you call this type of image in design term?
The closest formal term I know to something like this is a calligram which is
A word or piece of text in which the design and layout of the letters creates a visual image related to the meaning of the words themselves. -- Oxford definition
But most of the time calligrams use the text itself to create the shape of the larger word or image, not just lighting like in the image you provided. Using layout instead of solely text color makes more appealing and less thrown together.