I've been having a bit of a play with different colour choices for hydrogen emission nebulae. Here's a conventional rendition:
The Heart Nebula (IC 1805) - H-alpha + RGB
And here's a decidely unconventional rendition:
The Heart Nebula (IC 1805) - H-alpha, synthesised H-beta + RGB
The lilac colour is the same colour I see in a hydrogen discharge tube which is, unlike astronomical nebulae, bright enough to trigger the eye's colour vision. Whether the discharge tube actually emits the correct proportions of hydrogen's Balmer Series
(and nothing else) is a matter for debate on more specialist forums.