Could we accelerate plasma and shoot it at something by using something similar to a rail gun? Assuming that the plasma is ionized and has +/- charge (or is that not necessary?) If we did this what kind of visible effect would it have an the atmosphere? Could we see it?
Plasma beams are terrible weapons.
Plasma is ionized particles, atoms with electrons missing. Because it is electrically charged, it can be accelerated very rapidly to high speeds.
However, because all of the particles have the same kind of electric charge, it pushes against itself, and rapidly disperses into effectively zero density, so there is no punch by the time it hits the target.
There is ongoing research into ways to contain plasma in atmosphere use, but the effective range to date has still been very limited. For example, here is a video for a plasma gun with a range of 3 feet.
Plasma can have a number of different visual effects, Neon tubes (A neon plasma) has a characteristic orange, sodium yellow, and many other colors are possible.A lightning bolt can also be considered a plasma bolt. Neon, etc. are a result of the photons emitted when electrons drop to a lower evergy level, whereas the characteristic white flash of a lightning bolt is to due primarily to black body radiation at high temperature.
A lightning bolt is more coherent than a generic plasma weapon because the electric flow heats up the air until it ionizes, which make a relative good conducting channel so that the primary (return) stroke follows the same channel and does not spread out.
Any time you get the temperature high enough, you get a plasma. Some elements are easier to ionize than other.
Maybe with enough research, your could develop a practical plasma weapon for in-atmosphere use, but it won't be as easy as lasers or railguns.
Perhaps even more damning is that you can actually design effective shields against plasma weapon, since all you need are EM shields, much like the Earth's magnetic field is an effective shield against charged particles.