Hi Sean, Sorry for the late response! I will check notification settings.
By alpha I mean like a graphic alpha channel (transparency). In Pico-8 black is transparent.
In my case, I'm making a particle explosion and using shift-e and shift-b in the function FILLP(E) or FILLP(B) (where it shows the dotted pattern in place of E and B, with the built in patterns; which use color 0 as an alpha, and my chosen color as the fill pattern.
In your red/yellow example above, if the Red was color 0 (black), it would be transparent and you would only see yellow diagonal lines. Any circle filled with that pattern would show the yellow lines and what isn't yellow would show the game graphics under the fill where the color 0 is.
I'm new to programming but I did a lot of graphic work, so I might not be explaining it well. I thought this would help me fade out a filled circle by changing through a series of patterns with less and less coverage. That's what I'm doing now with the Shift-E and Shift-B, but I only get 2 steps of fade where I'd like about 5 or 7.
Again, I love this tool, I can't thank you enough for your contributions to the community!!! I appreciate it!