One small step for man; one leap second for computer systems?
I don't think its such a bad idea. I have to ask, would people really assume they are "under attack from little green men from outer space"? or "the gods are angry with them"? It's not first thing that would come to mind when I think of seeing the local area light up with bright light. That seems very 1950s, and extremely unreasonable.
Having the local area brighten suddenly, I would look around and then look up to search for the reason behind the sudden increase in luminance. what may be the reason for it before making assumptions like that. Measuring a tsunami is certainly feasible, even with low wavelengths, a tsunami is still a tsunami a excessively large mass of moving water in a series of waves. Instead of interfering with local emergency systems, a tsunami warning system as this could be used to complement local emergency systems by serving as primary warning.
Using different colours for the warning system would be pointless because until each you advertised each of the specific meanings to literally everyone, it would waste time for people to try to figure out what it meant. The biggest problem is powering such a system. (Perhaps you should get N.A.S.A.'s ideas for that part?)
On a point made by GrpA, if a person stopped to check a website to figure out if their area is about to be hit by a tsunami and one were on its way, then by the time they got outside, they'd be as good as dead.
"Do not go to look for a tsunami - if you are close enough to see one, you are too close to escape it unless above its level." (Wiseman, J (2004). SAS Survival Guide)