Nice game! Managed to get S rank somehow on my first try, it helped to realize each color was its specific kind of loop, and that one curve in a column of non-curve was a dead end.
I would have loved a right-click to turn the other way, even if that's only useful on the red cells :P
Small question, does a longer loop count containing both sides of a purple cell count for more points than the two shorter loops if the cell was rotated?