Blue Dragon:

Requirements:
Alignment - L, N, C
Water Dragon

Green Dragon:

Requirements:
Alignment - L, N, C
Earth Dragon

Purple Dragon:

Requirements:
Alignment - L, N, C
Wind Dragon

Red Dragon:

Requirements:
Alignment - L, N, C
Fire Dragon

White Dragon:
White dragons are the next step for any of the four regular dragons. White dragons advance in their stats much faster than the normal dragons, so change them to white as soon as possible.

Requirements:
Alignment - L, N
HP Max - Greater than 170
Str - Greater than 135
Vit - Greater than 90
Dex - Greater than 55