use whatever sign color looks best with the color wire you are using, doesn't really matter

using speaker (or whatever) wire on the back saves EL wire, but is a lot more soldering, which is a delicate procedure, but not too bad if you good at soldering TINY wires. If you do use EL on the back cover it with shrink wrap or black tape.

Before you get too far along, I would suggest testing some out....

Unless they are going to turn off the stadium lights for the ceremonies, I suspect that the ambient light will completely drown out the wire of your sign.

That said, assuming a darkened stadium.....paint the board black. As for the 'backside' either way will work. Merely covering it with black fabric is less trouble than soldering in speakerwire but depending on your sign, the amount of excess cool neon in the back will effect your cost, and possibly which driver you are able to use. The most efficient way to do it may also be a hybrid approach where you break things up by word or by line. So that letters that are close together are a continuous piece of wire but letters (or a group) of letters that is further might be connected via speaker wire.

Really, it just comes down to a matter of choice, and budget/time associated with the project.

Hope this helps!!

If it is dark, the sign will look nice, but won't be very visible from far away (e.g. across the stadium because the wire is thin.)

Here's a low-tech idea: take black foamcore. Paint a message in fluorescent paint. Bring along a 24" 12v fluourescent blacklight running off of a 12v 7amp-hour battery (an emergency light gel cell battery) and position it about a foot in front of the sign using a piece of wood to hold both. You can buy 12v fluorescent fixtures at stores that sell supplies for boats (e.g. West Marine or some such). Then just insert a blacklight tube.

It will be very bright and if you draw thick letters it will be visible from much farther away. Depending on the size of the sign you were planning to make, it could be much cheaper than the equivalent amount of coolneon, inverters and whatever batteries you were planning to use to light it. It also requires far less soldering. The bulb will be fragile. The rest is pretty durable, and it is easy to put a different sign in place; whereas, changing your coolneon sign will probably be difficult. Coolneon is fragile and it won't like being threaded and unthreaded from the sign again and again.