Firehawk, I had this problem too. You are missing that a blue sum only adds up the blue digits on that column/row. So, a blue 8 and a white of any number only equal 8.
I greatly enjoyed this game. I find it has a nice learning curve, it doesn't suddenly get too challenging, or too difficult. The music is not obnoxious, yet not distracting either, thanks to the undoing, which you will inevitably need to do. As the many others that have mentioned it, the fact that this game is not timed or limited on movements gives me even more appreciation for it. There are a couple of levels that make you want to just quit, but then you just find the right move and you feel so much better for accomplishing the challenge.
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Great comment! Thanks so much!! :D