I've never seen this before, that's why I'm reporting. I remember that bombs type mobs usually fell to the ground when defeated and kept the same colour until they became transparent and disappeared. Maybe it's just a matter of night time though. Anyways, it sounded odd for me.
Feel free to close this thread if this is working as intended.

Platform: PC
ISP: Movistar
Type of Internet Connection: ADSL
Internet Connection Speed: 10Mb/s
Date & Time: 07/27/2011 17:53 (GMT+2)
Frequency: Always
Character Name: Umo Heima
Race: Hyur Highlander
World: Gysahl
Class/Skill Rank: Conjurer/47
Physical Level: 50
Area and Coordinates: Eastern Thanalan > Halatali (around 42, 27)
Party or Solo: Solo
NPC Name: -
Monster Name: Brimstone Bomb
In-Game Time: 1:01
Steps:
1. Start "Preventing the Plague" guildleve at Halatali's aetheryte.
2. Go to designated spots (white light on the ground) and use "Examine".
3. Eventually spawned a Brimstone Bomb.
4. Defeated Brimstone Bomb.
5. Monster fell to the ground and its colour turned to a very dark and solid mix of blue and red. I've never seen this before. SCREENSHOT.