If google finds some bad techniques such as a same colour font and background, cloaking, etc. Will it ban me for ever, or is there a period where they'll come crawl me again and approve me?
you will not be banned forever. You will need to fix the problems and then send them an email asking to be considered for reinclusion. Then you have to wait.
Yes, Google will often ban a Website and re-index it when it's straightens out its act. This is true of many popular Website like PubSub and Wordpress who were banned for malpractices. If you are some small Website, then you will definitely not get the same attention and you may end up banned for a considerable period, possibly forever.