As mentioned by Spock in Undiscovered Country:
Under impulse power she expends
fuel like any other ship. We call
it "Plasma" - I do not know the
Klingon name for it, but by any
name it is merely ionized gas.
The Impulse Drives are a combination of Fusion Reactor, Drive Coil, and a vectored nozzle for plasma exhaust. As those technologies shift and advance the properties of the plasma exhaust that provide the thrust will change. We know that the visible color of an object is related to the atoms that make up the object and the temperature of the object. As the impulse reactors are fusion reactors that use Deuterium a heavy isotope of hydrogen they change color along the light spectrum depending on temperature of the fusion reaction. Red indicates a cooler, less energetic reaction, blue is hotter. This is seen in astronomy where hotter stars give off blue light, and cooler stars give off red (baring color shifts due to relative motion).
The NX-01 was early technology, probably using a primitive drive coil and accelerator-generator technology. As thrust is the flow rate of exhaust and the speed of the exhaust, hotter more energetic exhaust will provide more thrust. So on the NX-01 they run the fusion reactor hotter than later ships to get more thrust out of them, hence the higher frequency color of the plasma. As Drive coil technology improves and the ship can get more thrust out of cooler plasma, the fusion reactors run cooler, hence the lower frequency red impulse drives. An a cooler fusion reaction is safer, more fuel efficient, and easier to control and maintain.
As for the Accelerator-Generator mentioned in the memory alpha article, I assume that is what they use to shunt plasma from the reactor to either the drive coil or the EPS conduits as needed. As far as I can tell they are not mentioned by this name outside of the tech manuals. It may be another name for the impulse Relays which relay power from the impulse reactors to critical systems.