Black and white, sepia or a negative
Black and white turns every colour into a shade of grey and keeps the light and shade of the photo. Sepia does the same in warm brown tones, the look of old prints. A negative swaps light and dark, like the film that came out of a camera; with sepia, the negative keeps the brown tones.
How colours become shades of grey
The eye sees green as brighter than red, and blue as the darkest of the three. So the tool does not simply average the colours: it counts green the most and blue the least, close to the way the eye weighs them (about 59% green, 30% red and 11% blue). A red jacket and a green plant of the same strength therefore turn into different greys, as they would on black and white film.
When to change the contrast
A photo without its colours can look flat, because two colours that stood apart may turn into the same grey. Raise the contrast to make the blacks deeper and the whites brighter. Lower it for a softer, quieter picture. Watch the brightest parts as you go: when they turn pure white, the detail in them is lost.
Black and white headshots
A black and white headshot looks timeless and draws the eye to the face. It also hides what colour gives away: a busy background, a colour cast from the light, or clothes that clash. For a team page, black and white makes photos taken in different places look like one set.
Your photo stays the same size
The picture keeps all its pixels: the tool changes the colours, not the size. It is saved in the format of your file, or as JPG, PNG or WebP if you pick one under More options. A PNG keeps its see-through parts.