Is $E=mc^2$ not just $E=m$. What does the speed of light have to do with this other than to give it a really big number so it looks cool? What spectrum of light is used? How can we test the speed of light with out a stationary point to test it from?
The speed of light is there for much more than to look cool, and in fact there are a number of derivations of mass-energy equivalence that shows why $c$ is present; I will say that one basic reason is that the units of mass and energy are different, so we require at least some sort of constant factor to make the units work. I'll also say that we often use units where $c = 1$, making $E = m$ true; this is, however, separate from the question you're asking.
The spectrum of light is irrelevant, as all light moves at the same speed $c$, as can be shown from Maxwell's equations. Furthermore, $c$ can be calculated from two fundamental constants: the vacuum permittivity constant $\epsilon_0$ and the vacuum permeability constant $\mu_0$. These constants are the the same in every reference frame, and so the speed of light must be the same in every reference frame, as per the postulates of the theory of relativity. Changing reference frames only changes the apparent frequency of light, that is to say, its location in the spectrum. This is what we call red/blueshifting.
The speed of light in a vacuum is invariant: it is the same no matter what point you pick as "stationary". So if I'm on a train, and you're on the ground, and we both measure $c$, we'll get exactly the same number.
The speed of light does not depend on the wavelength. Gamma rays travel at the same speed $c$ as radio waves. The frequency $f$ and wavelength $\lambda$ change according to $c = \lambda f$.
The fact that the speed of light is invariant leads to a long chain of implications - along the way comes $E = mc^2$. The presence of $c$ is not just for making it look cool, but actually a necessary consequence of special relativity.
One shoul think as c as a kind of space-time conversion constat, massless energy travel at this speed. Light and gravity are kinds of massless energy. The idea of E=mc^2 is that mass converts to energy like this.