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One question: are they possible?

I'm not entirely sure about rocket and grenade trails but it is possible to change the colour of a rail particle using A_CustomRailgun.

I was looking at A_CustomRailgun, the problem is that it needs to be able to bend

Heh, maybe you could define the colour with the use of the GRENADETRAIL flag? Unfortunately the wiki doesn't say anything about colour though. I would've said ROCKETTRAIL with that but it appears to only be a Strife flag.

A generic "particle spawner" would give me exactly what I need. Basically I'm after a version of the rocket trail which is red instead of yellow.

Particles are woefully hard to customize. They're in need of a modernization to give them more flexibility.