Why Espruino did not suppert rgba color?

If you're creating something with Graphics.createArrayBuffer then yes you can read pixels back, and actually it's already done when drawing antialiased lines/polys/fonts.

So in that case, RGBA could be added. It's just not been worth the extra overhead (it might end up making normal draws slower) when I don't think it would get used much - it's extremely rare you have a device with enough RAM to store a decent size RGB image.