The greens can be tricky but there is a logic. On the grid, blue means upward and red means downward. For side to side, the faster the dots are moving (they always move downward) the steeper the drop. It's also very useful to use the Reverse view (lower right corner of the page, or Space Bar) to get a perspective from the hole back to you. You can put your mouse pointer on the arrow, hold down the left button and move it in whatever direction you want to to compensate. Remember you'll also have to adjust your stroke to accomodate upward or downward slopes. To get feel for relative putt strength, look around in the forums for a very nice description posted by nivlac.