White needle is indicated airspeed - based on air density. Red needle is true airspeed. Up that high the air is thin, so your white needle will show you flying a lot slower than you acutally are. Pay attention to the white needle, as it is what helps you determine how aggressive you can jerk the plane around without stalling.
If your plane easily stalls at 5k while flying 180mph and turning hard, at 25k you might be going 400 but the thin air makes it seem like you're flying closer to 180 and your plane will react accordingly.
That's my limited, non-scientific opinion as a cartoon pilot.