The chromatic dragons in the original trilogy where specifically designed to be different from the normal 1e chromatics. IIRC, which ever artist first designed a dragon set the standard for that dragon. It is easiest to see in the classic Takhisis art by Caldwell and the 1e MM:
1e Takhisis
View attachment 268621
1e Tiamat:
View attachment 268625
Black Dragon: two nose horns instead of forward pointing bull horns
Blue: no nose horn
Red: most similiar, but added more horns/spikes
Green: no dorsal frill and added horns
White: a crown of horns vs single horn/frill
These were intentionally different than the MM versions.