Can this sign be extended to make it longer?

From a hardware perspective, yes, the circuitry can easily be expanded. However, the included microcontroller is programmed only for a display length of 24 pixels. If you wrote your own firmware you could make it work.

Can I use bigger LED matrices and/or different colors?

Yes, as long as you get the pinout right you can wire up larger LED matrices (we sell some in our Electronics Components section. When using anything but the included displays (especially if changing color) you may need to alter the LED voltage by changing one of the resistors on the LM317 regulator. Be careful about asking too much of the sign - if the LM317 gets too hot you need to reduce the input voltage and/or the LED voltage.

Will you supply the source code or hex file?

No, we have decided not to supply this in order to protect our product, but we do provide an example assembly code file.