Looking at it again, it looks different enough from the prismatic. So without further ado...
Rainbow Dragon
Created by: Gregory Detwiler
FREQUENCY: Very rare
NO. APPEARING: 1 (5% of 2)
ARMOR CLASS: -2
MOVE: 9"/24" (MC: E)
HIT DICE: 10-12
% IN LAIR: 60%
TREASURE TYPE: H, S, T, U
NO. OF ATTACKS: 2 claws and 1 bite
DAMAGE/ATTACK: 1-B/1-8/3-30
SPECIAL ATTACKS: Major breath weapon, exceptional damage from bite, night blinding, spell use
SPECIAL DEFENSES: Sun dazzle, low armor class, high intelligence, detect invisible/hidden beings, saving- throw bonus with age
MAGIC RESISTANCE: Standard
INTELLIGENCE: Exceptional
ALIGNMENT: Neutral evil
SIZE: L (50' long)
PSIONIC ABILITY: Nil
CHANCE OF:
Speaking: 100%
Magic Use: 100%
Sleeping: 20%
LEVEL/XP VALUE: 10 HD: IX/6,750 + 14 per hit point; 11-12 HD: IX/8,050 per hit point
The rainbow dragon is one of the most powerful of all evil-dragons. It is also the most beautiful, its hide displaying an everchanging display of iridescent hues of all colors. When the dragon is in bright desert light, this dazzling play of colors gleams so brightly that it is painful to the eyes, causing any attackers to attack at -2 to hit, or -4 in addition to penalties for fighting in the light if the attackers (like orcs or drow) are not accustomed to direct sunlight.
The rainbow dragon usually fights with a claw/claw/bite routine. Five times per day, it can use its breath weapon of pure energy, which looks like a rainbow-hued blast of light. This weapon does damage equal to the dragon's hit points, but affects only one target per attack. The breath weapon is a 5'-wide beam of energy that extends for 100'. So bright is this light that if the dragon attacks at night, everyone within 100' of the beam must save vs.
spells or be blinded for 1-4 rounds.
In addition to-this, all rainbow dragons are capable of spell use. Very young and young stages of growth allow the rainbow dragon to acquire one first-level spell at each stage; at subadult and young-adult stages each, it may acquire a second-level spell. A third-level spell is gained at both the adult and old stages, and a fourth-level spell at both the very old and ancient stages. All rainbow dragons can speak and enjoy negotiating for ever-larger tributes and payoffs from cities that wish to keep such dragons from attacking them.
Rainbow dragons aggressively patrol their territories (especially in daytime) and prefer to attack when the sun is out so as to take advantage of their light-enhancing defenses. Rainbow dragons prefer brightly lit desert and plains regions, making their lairs in old ruins or caverns.
The blood of these beasts can be used as ingredients in scroll ink for the spells color spray, prismatic sphere, prismatic spray, and prismatic wall. A bit of bone from a rainbow dragon is said to tip the best wands of conjuration. Attacks based on elemental sources (earth, air, fire, and water) affect the rainbow dragon normally. Electrical attacks are taken with a - 1 hp/HD penalty to damage, which is helpful if the rainbow dragon fights a blue dragon for the same territory.
Source: Dragon #146, "The Dragon's Bestiary", Gregory Detwiler.