How to implement RGBA in 8-bit microcontroller?

Question

I have successfully connected my microcontroller to my TV via through VGA adapter i made that convert digital to analog signals with the help of resistors.

Currently i am utilizing 8 pins that I am using as 2 Red 2 Green 2 Blue 1 Hsyn and 1 Vsyn. I was thinking if there is a way to add 'A' Alpha in it, it's used for transparency. My another question is that is Alpha software driven or is it in the hardware?

Apparently, i did not find any relevant resources on hardware side of RGBA. I did notice Gameduino has used it in their 16-bit microcontroller. Is it possible to use multiplexing/charlieplexing in this problem?