Hello,
In my experience as a Reiki Master, every single session with a client (or myself) is different. So if you continue your sessions, they will always be different. We are dynamic beings, always changing, always working toward balance. Just as in meditation, some sessions will be blissful and some will stir things up that surface for release. This can be uncomfortable, but better out than staying in the tissue at the cellular level. Your compulsion to get up off the table was probably resistance to some emotional release. Stay with the emotion when that happens, and you will move through it. If it doesn't happen during the session, release can happen during the week or so after, as you discharge old toxic emotions, have an important dream, gain insight about a particular issue, recall a repressed memory, etc. The session doesn't end when you get off the table. You continue integrating the energy for several days afterward.
Experiencing colors is very common as you begin the energy journey. It's fun, isn't it?
As a practitioner, I never tell a client that I encounter "negative" emotions. Emotions/feelings are neither right or wrong, they just are. There ARE NO negative emotions. We need to honor all of our emotions, and observe them with interest and curiosity. They often lead us to the very root of our main issues. Allow them to surface, thank them for serving you in the past, and bless them on their way as they are released. (An emotional release may be only one layer of the onion, but it is a start). I have had years of bodywork and I am still releasing old patterns.
Also, the practitioner does not "cleanse" anything. All healing comes from within. You are the healer, not the practitioner. She/he may be channeling/facilitating the energy, but your will decides what to do with it.
Oh, and the headache can be a sign of detox. When you start this journey, anything that is not in balance needs to be cleared.
Have fun with Reiki, it is a beautiful, simple modality that a child can learn to do.