With the Camera app, you can add a filter which will appear live in the viewfinder while taking a picture.

In either the Photo or Square mode, tap the filters icon (three overlapping circles in the top right corner) to reveal a selection of filters. These will be at the bottom of the screen, or on the side if you’re shooting in landscape orientation. You can swipe through to preview each filter, and then take the shot when you’re ready.

While a filter is enabled, the filters icon becomes illuminated. Tap this and change the filter back to Original if you want to turn it off.