How do I use alpha values in material base colors, without image textures?

I do a lot of rendering where I render an image of a car with its body on, then another image with its body off, then use imagemagick to average the two to be able to see both at once. I’ve been trying to figure out how to do this with a transparent material without an image texture, and can’t figure it out. I change the alpha value of the base color, and it does nothing. Why not?

Do you have transparency enabled?

I didn’t, thanks, that did something, but… I created a new .blend file, with two cubes, one transparent in front of the other, with blend mode: alpha blend. And it looks different, but I don’t see the other cube behind it.

What I’m roughly trying to accomplish:

+

=

Do it like this

Thank you very much. That works in a new test blend file, but not in the one I’ve been working on for a long time. I’m editing them with the same blender version. In the new test file, the options look the same as yours. In the one I care about, on a new object, with a new material, the Settings section does not contain Blend Mode:

That’s because you are using Cycles as your renderengine apparently which doesn’t need that option… it should just work out of the box usually(?) If it doesn’t, there is something weird going on - even in the Preview of your screenshot you see transparency.

The default engine now is Eevee - a realtime Render Engine faster than Cycles but also much less acurate.

Hah, I swear I tried the same thing again, just changing that alpha setting, and it worked! Thank you so much. One render:

Yes, I’m using cycles, because I haven’t figured out how to achieve results I like with evee.

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i did atest in 2.9 and in render mode one cube is tranmsp with PBR alpha

but not in the viewport !

so is there a way to see transp in viewport too with PBR alpha ?
may be using Workbench render but how !

thanks
happy bl