Roll up garage door spring ... unwound?
The paint marks are used as a general guide to how tightly the spring is wound.  In this case it's clear that the right spring is wound more tightly than the left one.  Yes, it's possible that the left one has slipped or they were not evenly wound to start with.
The two springs should be tensioned the same and this is done by counting the turns as you wind them.  The tension should be set so that the door is roughly &quot;neutral&quot;.  By that I mean that it can be moved up and down with minimal force from either the opener or from a manual operator if the opener is disconnected.  At the top it should stay up when properly tensioned.
You are correct, however, that tensioning these springs can be very dangerous!!  But using proper tools and methods and wearing safety glasses you can minimize the danger of personal injury.
Using the winding tool that fits snugly into the sockets on the hub, you take the tension off the setting bolt and then loosen it.  Now using one of the tools (you will need two of them) you can wind or unwind it from about 1/4 to 1/2 a turn.  Repeat until you have tightened to the desired tension or loosened it fully.  DO NOT RELEASE THE HUB AND LET IT SPIN!!  That is VERY dangerous!
If you find you are unable to do this yourself safely, please call a qualified overhead door service tech.