If you are adding a caption on your image or on your video then you have to select text color very wisely so that it can be readable and don’t get mixed with color of background image. If you have been struggling hard to find perfect solution for this problem then here is a simple solution for you.
The one solution which is widely used on internet is that you can make text color white and make a black color outline but make out line thin as possible, 1px outline will do it for you. If you take over internet then most the web services uses this technique like if you enable caption on YouTube videos then you will find out that they are also using white color text with black color outline, Even Google+ also sues same thing.
One more thing, try to use Arial or Verdana as your font for caption on video or images as they are easy to read and widely used. Its simple and easy to use thing which you can use to make your caption more readable.