Why did some of my Nikon D7100 photos save as black and white when no monochrome settings were selected? I realized later it was shot in "effects" mode but some of the photos saved black and white and some did not. I want them in color. Is there a way to reverse this?
On the D7100, there is many way of influencing color of the photo you take. The 2 main ones are Color Profiles (standard, vivid, neutral...) and Special Effects.
The Special effects can be used by turning the mode dial to 'effects'. Then the desired special effect can be selected by the main command dial. Some special effects can turn the image Black & White such as Night Vision or Selective Color. When you take a photo the special effect is applied on top of the color profile selected then stored in memory.
That means there is no way to recover the color version of your photos taken in effects mode.
To avoid this, you must not use the 'effects' mode directly. Prefer the green auto mode or P mode. If you like the Special Effects, you can always create a copy of the photo and retouch it in-camera, see Nikon D7100 User Manual page 274.
Even in auto mode, you will still have the color profile automatically applied to your photos among other things like white balance... Once a photo is taken, its colors cannot be changed. You can retouch them lightly in camera. However, you will notice that heavy retouching in-camera can degrade the image quality.
To have a total control on the colors and the quality of your photos, consider shooting RAW or RAW+JPEG (see image quality page 66) and learning to process RAW files on a dedicated photo editor like Lightroom(paid), CaptureOne(paid), Darktable(free), RawTherapee(free)... Choose one and learn it. You will find plenty of resources to learn them online.
Also, processing your photos this way allows you to not think about colors during shooting and concentrate on your subject more or be less hesitant shooting. Later behind the computer, you can have a deeper though about which effect or color you want to achieve.