You may have fixed the problem, but here’s some info:
RGBA stands for Red-Green-Blue-Alpha. The RGB part dictates the colors based on combinations of Red, Green, and Blue channels. If you look at one channel by itself, it will appear black and white, where white means 100% “strength” and black means 0% “strength”. If you had a red channel that is all white, while the green and blue channel were all black, the final color would just be red.
Alpha is also in black and white, except it’s used to measure opacity/translucency of a texture, where white means 100% visible and black means 100% invisible.
Source has some annoying VMT issues. Like the one you experienced with using $selfillum. #selfillum is used to dictate whether or not parts if the texture glow (looking like they produce light). $selfillum uses the Alpha channel to tell what part of the texture is glowing (white means glowing, black means not glowing). Using both #translucent and $selfillum tells the game to use the alpha channel what is both see through and what glows. In your case, the visor was translucent, but also produced a glow that made it visible again.
The visor doesn’t seem like it needs to glow anywhere, so you should be fine with leaving out $selfillum. In the future, you’d be better off having the main texture’s Alpha channel define the transparency of the model, and create a separate texture to define parts that glow on the model.