Golds and Reds will develop more colour as they grow, as most start out quite plain and dull white as juveniles. Greens are much the same, and you really don't know what you're in for until they grow into adults. I've got a 6-7" Green Severum and she's gone from pretty much black, to a dull creamy greeny yellow for a couple of months and is just beginning to darken back up again. Severums also vary their colours a lot depending on mood.
They are sexually dimorphic. It's not always 100% accurate, but Males have the dark facial worms on their operculums (gill covers) and between their eyes, as well as growing slightly larger and having very extended dorsal, pelvic and anal fin tips. Females can also have the facial worms but they're often not as vivid or complex.
Also know that most Severums in the hobby are not Heros severus. They're generally Heros efasciatus.