When blocks can be moved to a partially filled tube, it is highlighted. When there is no partially filled tube to match, any colour can move to an empty tube, if you have one.
After you make the first move, try undoing to see the borders! A double line around a tube indicates that move goes to a state you have already visited. In this case since all the tubes are the same each initial move is equivelent, but in normal games the double border is a good way to identify what you have already tried.
If a move would lead to a dead end it is given a red border, try clicking on it to see what a deadend looks like! Undo or reset is always an option.