As of this day, I'm programming my brain to always type "FireRed", "LeafGreen", "HeartGold", and "SoulSilver"... as that is how it's officially written (with no SPACE Character in-between the 2 words), even on Nintendo's websites. 

EXAMPLE: http://www.pokemon.com/#videogames_20 

In programming/.NET methods or even with SQL functions, they call this concatenation, if anyone wants to take away some trivia from this post. 

Anyway, if you wanna sound kewl, please say HeartGold and/or SoulSilver from now on... and not "Heart Gold" and/or "Soul Silver" :p

Nintendo's concatenations are ridiculous. Firered is not a word, it's a misspelling of "fired". :P
FireRed, LeafGreen, HeartGold, and SoulSilver sound like colors that were created for use on crayons. :D

I like that whenever I (hopefully) refer lovingly to SoulSilver in the future while abbreviating it as SS, I'll feel like a Nazi.