In studies of advanced fusion spacecraft the engine exhaust is often depicted as a white jet of collimated gas emitted from the rear of the vehicle. A case in point is the Daedalus study, a theoretical research project into an interstellar flyby probe conducted by the British Interplanetary Society in the 1970s. In almost all of the depictions it is shown with a very bright white emitting jet from the engine. An example of this is shown in the beautiful image below by Nick Stevens. The powerful engine associated with this vehicle would have a thrust of around 7.5 MN and an associated jet power of 40 TW, releasing energy at a rate of 4.42*10^13 J every second.
Illustration of Daedalus by Nick Stevens
Yet, realistically what would the colour of the exhaust plume be and would you even be able to see it. This is worth some examination. The table below shows several calculations for key fusion reactions that might be expected to be utilised within an advanced fusion engine. The last one is the proton-proton reaction which is very difficult to ignite due to its incredibly small cross section, except at the centre of the Sun where it is achieved thanks to the massive gravitational pull.
Also shown in the table is the expected frequency range of the total energy emission. This is then translated to a wavelength for the radiation. It is shown that the wavelength range is given as 20.6 - 123.5 pm, where 1 pico = 10^-12. To write this another way this is a range of 2.0610^-11 m (0.02 nm) to 1.23510^-10 m (0.1235 nm). Then looking at these wavelength range on any electromagnetic spectrum it is shown that they correspond to x-rays.
These x-rays are generated from several physics processes within a fusion engine. This may be from Bremsstrahlung radiation, Synchrotron radiation (relativistic) or Cyclotron radiation (non-relativistic) and it really depends on the type of fusion engine being used, i.e. magnetic bottle trapping versus laser induced inertial confinement fusion.
For the wavelengths calculated above, these x-rays are outside of the visible part of the electromagnetic spectrum. This means that a lot of it would not be visible to the naked eye, although it could be detected in x-rays detectors. That said, its possible that some of the emission would approach ~100 nm range which begins to tend towards the extreme ultraviolet. Even though there is less of this light being emitted, compared to those in the x-rays, it will be incredibly bright from our perspective, and would tend towards a light purple colour - perhaps violet.
So this then might be a recommendation for any artist depicting an advanced fusion propulsion engine, to perhaps paint the jet exhaust lightly with a tint of purple/violet haze within it. This might not look as spectacular as say the images like the one shown above, but it would be more realistic. That said, any humans looking at this light would have to be a a very safe distance and also behind an x-ray shielding screen, since the dose of radiation from the jet emission would be too dangerous to observe directly.
This also means that launching any such vehicles would have to be away from Earth orbit with its orbiting satellites and human crewed space stations or we can expect much damage to occur. The vehicle would have to be ‘tugged’ out first to a safe distance before switching on the engine. Although astronomers from Earth would be able to observe it through their detectors speeding off towards infinity and that would make for some good images.