Green block means happy block. Once it turns into yellow color, it becomes unhappy. How to make it happy again? Hm, you need a green block which is spreading happiness all around. Even then it is not so easy to make them all happy. Blocks are placed in certain spots which can be accessed from certain positions, so you cannot just randomly place green blocks and hope for success. You must think ahead and even if you fail for the first time, do not give up and try it again! Have fun.