oops, thinking about it a bit more (well, not very well - it's 4.30am here in London), Hydrogen is red in space I think because its the main emission/absorption wavelength of the molecule - not because of scattering of white light. You'd probably need a lot of it in the atmosphere to see this effect....Drat!
As for the reason Hydrogen escaping Earth's gravity. It is that at room temperature (well to be frank, most temperatures) the average speed of a hydrogen molecule is faster than the esacpe velocity of the planet - so it effectively shoots off into space with no problems whatsoever. You'd need a heavy Earth to hold on to it - don't know how massive it would have to be though, could work it out very approximately on a scribble of paper in the morning...
The other problem is that with a planet with oxygen, lots of Hydrogen would probably burn with it quite nicely!
To get a nice range of scattering you'd probably need quite a large molecule in the air - something considerably bigger than oxygen - which I'm assuming will have to be there for humans to breath. A dry dusty world could kick up a large amout of big particles that just won't be washed out of the atmosphere - like Mars. But big molecules in the atmosphere must be replenished otherwise they will eventually hit the surface again.....
I will ponder this over a quick snooze and get back to you
