For the purpose of this question assume that the problems associated with antimatter drives have been solved.
Let's say my spaceship is using a Matter-Antimatter rocket that reacts Antihydrogen with Hydrogen particles, what color would the exhaust or plume be?
As noted in the comments - there may be nothing to see.
If your magic antimatter rocket is just slapping matter and anti matter together and defeating physics to get all the resulting products traveling out the back you will just have a mess of high energy particles and photons that will be only be visible if the hit something. If they are hitting you there will be various visible effects to enjoy while being irradiated. Outside the stream there may be little or no human visible artifacts off axis other than whatever blackbody radiation the engine assembly itself produces.
If you are using the antimatter to drive some form of thermal propulsion then it will look much like a more boring conventional rocket. If exhausting pure hydrogen it will probably be producing a red glow unless really odd physics is happening. the glow will be less exciting to watch than a chemical rocket since there is no chemical reactions glowing or carbon soot radiating black body. The really interesting physics will be happening inside the rocket, and the energy levels limited to those similar to a chemical rocket by the same need to not melt the engine assembly.
If for some reason you NEED a glowing rocket exhaust then you can inject chemicals into the exhaust stream for a wide range of emission spectra. If this is for a work of fiction than you can argue for pretty much any colour your plot requires coming from trace elements from the engine or fuel.
