For mobile, the best approach I can come up with to handle having more pairs of nodes than easily distinguishable colors would be:
Have two ways of moving around. You can drag from one node to another (the fastest way to do things if a level is not very complicated or already nearly solved) and have both nodes be highlighted as soon as you start the drag from one.
Or you can tap and release a node which would select that node, highlight both nodes of the pair, and put you in a step-by-step movement mode. The four edges of the screen would have movement arrow buttons so you can make a line from the starting node one step at a time (helps with navigating tight turns on a small device), or you can tap the starting node again (or anything except movement arrows) to cancel the selection without having it count as a turn. That way you could tap and cancel to highlight the pair of ends temporarily so you know where they are. (Or instead of arrows at the edge of the screen you could have the player swipe to take a step in that direction... whatever you think is most intuitive and easily playable.)
Another potentially nice feature would be a toggle button to either allow or prevent you from breaking any lines that have already been placed if you draw a new line over them -- a fair number of times I've inadvertently broken a line by having a new line cross into it while I was drawing lines with a mouse, and that might get even tougher with a small mobile screen.
3p0ch
For mobile, the best approach I can come up with to handle having more pairs of nodes than easily distinguishable colors would be:
Have two ways of moving around. You can drag from one node to another (the fastest way to do things if a level is not very complicated or already nearly solved) and have both nodes be highlighted as soon as you start the drag from one.
Or you can tap and release a node which would select that node, highlight both nodes of the pair, and put you in a step-by-step movement mode. The four edges of the screen would have movement arrow buttons so you can make a line from the starting node one step at a time (helps with navigating tight turns on a small device), or you can tap the starting node again (or anything except movement arrows) to cancel the selection without having it count as a turn. That way you could tap and cancel to highlight the pair of ends temporarily so you know where they are. (Or instead of arrows at the edge of the screen you could have the player swipe to take a step in that direction... whatever you think is most intuitive and easily playable.)
Another potentially nice feature would be a toggle button to either allow or prevent you from breaking any lines that have already been placed if you draw a new line over them -- a fair number of times I've inadvertently broken a line by having a new line cross into it while I was drawing lines with a mouse, and that might get even tougher with a small mobile screen.
Good luck with the mobile version!