There are at least three indications that stress is being dissolved or released in meditation: 1. Movement of the Mind, 2. Movement of Emotions, and 3. Movement in the Body.

The movement of the mind in meditation, whether indicated by visual or verbally constructed thoughts, is an indication that stress is being dissolved or released. Mental activity can be visual such as seeing lights, colors, faces, images, scenes, etc. Other times you can have a creative idea, or your imagination kicks in, or you work on an endless to-do list. We certainly like some thoughts better than others, but rather than encouraging them, it's best to return your attention back to the focus of your meditation.

Thoughts in meditation are often about ordinary, current experiences, and are usually quickly forgotten after meditation. It's good to know that if they thought is really important, it will usually be there after the meditation period. It's also important to note that In meditation, the content of the thoughts in meditation often does not relate to the content of the stress being released. For instance, you could be thinking about lunch, but you're releasing stress from another experience.
 

Some people love all of this mental activity and turn their meditation period into a contemplation period. However, for optimum results, it's suggested not to try to make anything (thoughts, etc.) happen in meditation, and not to investigate or analyze why you are having the thoughts. You can do that when you are not meditating!

Now there are some people who equate colors with subtle energy centers in the body, energy centers called chakras. A chakra, which means wheel in Sanskrit, is a non-physical junction point where energy meets matter (chakras are often related to the major endocrine glands such as adrenals, testes and ovaries, pancreas, thymus, thyroid, pituitary, and pineal glands.) Some say that each chakra has a color associated with it too. So, colors in meditation can often indicate a release of stress related to the chakra that it represents, thus allowing for a freer spinning chakra and more life energy. If you would like to read about the colors, visit this link I found online.

There is, for every meditator, a choice point. A moment when you notice thoughts, emotions, or sensations distracting you in meditation. At that time you have a choice to return to the meditation practice, or to let the thoughts hijack your attention. It's up to you. And it can happen more than once in a meditation for sure! It's important not to spend time analyzing why this or that is happening; instead, acknowledge you are distracted, and simply, without judgment, refocus on the meditation practice. This is how meditation trains your brain. Don't stop meditating just because you are distracted by thoughts; instead, stay with the practice to allow the stress to be eliminated.

And, here's a great suggestion attributed to Siddhartha Gautama, aka Buddha (500 BCE). Believe nothing, no matter where you read it or who say it, unless it agrees with your own reason and common sense. Always good advice.