Whatever the locals are doing. At RHV, the red line is what everyone does. The unwritten rule is to pull forward enough to pass the planes doing a runup, turn left and swing around until you're pointed at the tower (which happens to be in a convenient place for this). Run-up, then call tower as you pull forward and into line for the runway. It also puts you the same direction as taxi-backs and allows you to check final for traffic (which is somewhat important here despite the tower to ensure that you can cross the parallel runways in time, as there are no taxiways for the western runway). It works well, as it keeps folks basically correctly lined up and mostly into the wind. You'll occasionally get bigger aircraft run up at the opposite end of the area, though still pointing at the tower.
At PAO, another one of my home airports, blue is closer to the truth, though PAO has the advantage of marked runup T's with taxi lines. Getting it wrong would mean ignoring the ground markings. When it's busy, ground often tells you which T it wants you to run up at. At MRY, HAF, and APC, it tends to blue. MRY's run-up is so large, you could run up a 777 in it and still fit me and all my best friends. HAF and APC have run-ups at a right angle to the one in the picture, so blue just works better.
If I'm at an unfamiliar airport and I'm alone...who cares? I tend toward blue just 'cause it's shorter to taxi, though the way the planes are pointed in the diagram is highly unoptimal in any case. If I'm not alone, I'll take a clue from the other pilots. If I'm first, I'll just make sure that whatever I pick uses the space efficiently enough to allow a maximum number of planes to run up. And that I not block anyone heading for the runway, if possible.
I picked red, since RHV is home for me, but I could have legitimately picked any.