The Preset Effects panel has a number of contone and duotone effects - Black and White being the most obvious example.
Tap or click a photo to select it, then tap Photo Effects on the properties panel to see the effects.
You can customize the presets - change the brightness and contrast within the Adjustments panel. Change the light and dark colors of the contone either with a tap on the pencil icon - to take you to the Photo Colors panel.
Or select the Colored photo option in the Adjustments panel. If you un-check the box the contone is removed.
On the Photo Colors panel tap a color to change and then choose a color to contone your photo from the Theme colors - or create your own by tapping on the + button to open the color editor.
When coloring photos there are two colors you can set to contone the image - so just as a black & white image shows a continuous tone range from black to white, so you can select any Dark and Light color for the image. This is often called Contoning for the case where the start and end colors are the same hue (e.g. black and white, or sepia) or Duotone where the start and end colors are different (e.g. an image goes from light yellow to darker purple as in the example below).
A single tap on a chosen color patch will apply the color to the photo as the Light contone with the Dark contone remaining black. This is probably the most common scenario - so if you wanted a photo to be black and white for example then just tap the white color patch.
Once you have contoned a photo the color patch shown in the properties panel is diagonally split, like the group colors, with the 2 halves representing the light and dark contone values.
There’s also a Reset button to restore the original image.