It actually looks more purple than pink, but that's normal considering the light reflected from the planet is red and essentially passing through a blue filter (the atmosphere). If you want it to appear dark red, you will most likely have to photoshop it, unless you are willing to consider drastically altering the atmosphere. Terragen 2 uses realistic lighting principles and atmospheric light scattering. I don't know of any way to bypass these routines. One thing you might want to try is placing it higher up in the sky. The amount of atmosphere between the planet and the viewpoint is lesser the further it is from the horizon, and thus will have less of an effect on the light passing through it.
One more thing: Because of the realistic lighting, a planet is only as visible as the sunlight it reflects. Just like the moon. If you want them to be more visible, increase thier brightness by either making them paler or increasing thier saturation. The colors toward the paler end of the spectrum will be more visible. It is also important to remember that the atmosphere itself is an object, and thus can have significant effects on the appearance of things that are behind it. And putting the planet closer won't have any effect until you reach the atmosphere, at which point the effect of the atmosphere will be lessened as the distance between the planet and the viewpoint decreases, and thusly, the amount of intervening atmosphere.