Comets are an evil tile, they destroy some of our arguments for solving levels. But still they are not that difficult. The most important thing to know is, when do you need a path or when don’t you.
This gets a little harder when there are two or more colours. Think about when you do not need the red or yellow permanent tiles any more. Here you first need to go clockwise, eliminate the red tiles with the red comet and then go on to the yellow tile. The tile in the middle can be collected from both sides before activating the last comet.
The permanent tiles can destroy the corner argument. Have a closer look at this puzzle:
The first tiles are no problem. Here the permanent tile can simply be handled as a normal tile to collect the whole region. However the upper part cannot be cleared that easily. From the permanent tile on you need to either go to the tile on top or left and then proceed until the comet.