you look at it from the side, unless its more sensitive stuff like GH and KH tests. i find that with my KH test, I have to look down to see any color because when the KH is 0, the test water looks clearish so its hard to tell any color change apart from clear water. if you look down it actually looks orangey - meaning that it took even less than one drop to change from blue to orange as indicated by the KH test.
I only found this problem apparent with KH tests. others are simple enough to tell from the side of the tube. always be looking at it from the side against a white background (the color chart card).