In Gemcraft Chapter 0, which of the 8 gem colors are the most useful in traps? Are there some colors that are only worth it to put in towers?
Well, this depends on your playing style, but generally, any gem that has a long-lasting effect (i.e. Slowing and Poison) are better, because their Effect is enhanced.
Damage-causing gems become less effective, so Splash and Shock are not good choices for normal play. They could be a good choice on a swarm level, however.
And finally, if you are using Stealing gems primarily as a source of income, you would, of course, benefit in placing them in traps, rather than towers.
The RO gem collects mana from not only it's alien passer-by, but also others around it, thanks to the red splash effect. It is also capable of wiping out weak aliens that pass by. Orange, and Red-Orange gems are used for "Manna Farming"
Green gems are a great choice in traps. You can place traps with green gems all around the track to create a "Poison Trail". Poison doesn't stack (a poisoned creature that goes over a green gem again doesn't do anything), so it's not beneficial to put them next to one another. Poison trails have a new green-gem trap on spaces where the last poison effect wore off, to make your monsters continuously poisoned, so they drop dead before they reach your base.
Light Blue Gems
Light blue gems shock a creature, giving it damadge and freezing it in place for a few seconds. Durring this time, it doesn't move, so it can be hit many times by your towers. It's a great idea to place these next to your towers.
Blue gems have a simliar effect, only they slow the monsters down. Placing these near towers means the monsters slow down to take more hits by towers as they pass by.
Purple gems lower the armor (defense) of monsters, making them take more damage.
This really depends on how far you are in the game. Some endgame tactics focus on getting a really high lvl triple gem in a trap.
Personally I'd try to start out with a high lvl poison/green gem where the creep start. This will kill the first couple of waves enabling me to quickly summon them with extra monsters where needed. Ramping up lots of mana early on.
Later on I usually try to add a slow/blue gem close to my towers to slow down the mobs passing by.
Orange mana gems are great if you start out the level with some traps. Just put in some and let the mana flow.