You could do it with string (one piece twice or two pieces to mark the outline, then paint). Once you pick the angle and pull the string tight, it will define an even spiral. But there's an easy, one-step solution that gives you a perfect spiral without needing to paint. Use a piece of red ribbon the desired width of the stripe and wrap the tube at the desired angle. As long as the tape isn't stretchy, the starting angle will define a perfect spiral.
If it's not going to be handled, you only need to secure the ends. If you want it more secure, you can tape the ribbon edge. Or after figuring out the angle you want and the ribbon length you need, leave one end of the ribbon secured, unwind it, and use glue or double-sided tape on the back of the ribbon as you rewind it.
If you can't find red ribbon in the needed width, make your own. Cut strips of something weatherproof like Tyvek and glue or tape them end to end to create a ribbon of the needed length (just ensure that the ribbon edge is straight), and paint it red, or cut up a disposable red plastic table cloth.
Another option would be to figure out the angle with a piece of string or strip of paper, mark a starting line on the tube for the edge as a guide, then apply red tape to the tube at that angle. You can get tapes like duct tape or electrical tape in red. As long as you apply the tape flat, it will maintain the spiral. If you somehow stretch it so it comes out uneven, peel up the problem area and reapply it. If you use something like electrical tape, which is a little stretchy, use a paper strip against the previous edge of the tape as a guide to maintain the spacing.