Using the filters in iOS 7's Camera app is a fun way to spruce a photo. For whatever reason, iOS 7 doesn't deselect a filter after you exit the Camera app, leading you to inadvertently filter some photos.
For example, if you take a pic using any of the available filters, exit the app, and then awhile later quickly launch the camera to catch an impromptu photo, the filter is still selected and thus applied to your photo.
Luckily, there's a quick and easy way built into iOS 7 that allows you to change or remove filters from a photo.
To remove a filter from a photo, launch the Photos app on your iOS device. Then find and view the photo you want to work on, and tap Edit.
Once you tap Edit you'll see some editing tools along the bottom of the screen. Select the filter icon.
You're not only given the option to change the applied filter, but if you scroll to the beginning of the filter list you can also select None, which in turn removes any applied filter to a photo.
This tip was brought to my attention by a reader shortly after iOS 7 launched, but I just now got around to actually trying it out after recent a frustrating experience with filters. Thanks, Danvin!