I had some difficulties using Union in my knife blade model so I did some experiments with a simple minded model of two blocks.
First I translated, the lower left corner of the green block to the closest corner of the blue block. Then did a Union to make them one shape. Curious that the new Shape is of mixed colors with only one face changing to blue? I would expect a single color or that prior colors are maintained.
Not a big deal! But I know that you guys are perfectionists so I am reporting…
Actually, it should be blue. Or green. Those two faces are merged after you united them (since they are on the same plane), and currently you can set colors per face only. So one face can have one color, and those two faces become one after the union operation. So this is the expected behavior.
Daniel, I ran into the problem of not being able to Union what I thought were abutted pieces of my knife blade. So I translated one piece into the other by 0.005 in. And the Union then worked.
But I saw items in tutorials where one shape was translated to a point of another shape such that they had a common face and then a Union was done successfully. So this is why I did my test and it worked. I would hope that this would continue to work to avoid the hassle of going beyond just butting planar pieces together and doing a Union.
Istvan is correct in his logic about the color. Thanks for pointing this out.