Okay, as promised, here's how I do my Alpha textures using the base textures after a repaint. Excuse the bold and large text, this is for people like Helldiver and me and because the images are so large, I do not want them to hide or overwhelm the text.
First I start with my base layered repaint texture. Note that I like to keep layer sets, as it makes life much simpler when wanting to uncheck or check all layers. involved:
Once open, close all layer sets save the Paint and background (you want this as a guide at first, more on it later):
I open the Paint Layer Set and highlight the silver, which I will change the name to silver_alpha (double-click on the name and it will highlight and one can change the name). Next I choose a proper Alpha gray for silver, in this case a RBG of 175 overall. One must stay in grayscales from 255 down to 0. The base silver is clicked with the Magic Wand while holding Shift, so that one can do all areas, then paint the Alpha color chosen by Drop Bucket or the Paint Tool.:
Next I apply a dark alpha to instrument faces and bright silver objects. an RBG of 120 is chosen for this. Areas receiving the dark Alpha are selected using various Photoshop tools including the Select Ellipse Tool (hold Shift while selecting for circles in lieu of ellipses), Rectangle Select Tool, Magic Wand Tool, and Polygonal Lasso Tool, , then painted the color chose by Drop Bucket or brush after selected.:
There are a few areas that will need an Alpha white before I work in the details and markings, the tire and holes in the landing gear doors. This is one reason I leave the background open, as it is needed as a guide. First make a New Layer and name it Alpha_white. I use the Ellipse select Tool to mark out the tire and paint it pure white (RBG 255). For the door holes I use the sharp round brush tool with a value of 13 pixels for the large holes and 9 pixels for the small holes.
Now it time to paint the background totally black (RBG 0).:
This completes Part 1 or the tutorial, as I must hold the quantity per post down. Part 2 will appear shortly and it shows how to get the Alpha white for all paint and markings in a jiffy.