Maybe you’ve turned on Use Part Color in properties? Try turning that of. Use Part Color makes your entire union one color, which can be useful at times, but not when trying to create a multicolored union.
You probably didn’t get it.
On the first picture, you can see those shapes I need to make holes with, but this will happen.
It is not about the colour.
Are you trying to negate (engrave/etch) those patterns into the wall? If so, you’ll have to first select the pattern and negate it. After doing so, then try unioning the pattern with the wall.
…
I know how to union, but you can see what is going to happen, It won’t make that shape.
It is not a z-fighting.
I’m not talking about z-fighting.
Do you want the final result to have the yellow pattern? To have the pattern’s shape in the wall? To remove the color?
If you’re trying to make the final result yellow or trying to remove the color then look at the first post.
If you’re trying to have the pattern’s shape in the wall then look at the second post.
I need that shape in the wall, but this is what is going to happend when I do on the second picture.
He’s trying to figure out what steps you are doing and what you are expecting to happen.
The first pic we see a gray wall with what looks like a yellow Union sticking out of it.
The second pic is hard to tell, looks like a gray wall with a messed up red Union sticking out.
If you want the yellow part sticking out, just make it a Union, make it CanCollide off, and make the wall a solid gray Part.
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If you want to keep the yellow, then union them together with Use Part Color in properties off.
I need to makes these shapes negative, then union it with the wall, so it will make hole of that shape. But this is going to happen…
If you want a hole all the way through, then extend the yellow sections until they pass through the grey wall.
Right now they’re too shallow and stop before they make it all the way through.
You have too many “triangles” in your union. Roblox has a limit on the amount of triangles you can have (I forgot the exact amount), and once you exceed that limit, it automatically “simplifies” your union for you, in other words it changes the shape for you.
You should probably union them by segment, like split that union into 5 segments or so, so you have 5 unions and not 1 union. That usually solves the problem for me, since then each union will have a smaller triangle count than one big union.
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Let me try! I will let you know, btw. You mean that wall or those shapes? (Union into 5 segments)
Just checked, I believe CSG (Unions) has a limit of 5k triangles. Meshes can be 10k. Consider using Blender to remake that, it will be a lot better optimization-wise too.
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Well, I’m still learning with blender, so It’s not possible for me to do that at the moment.
Alright, then the best solution would be to split your union into smaller unions. Get back to me on how it goes! 
So is it fine now?
I fit still looks weird you need to make the amount of parts you union smaller again.
PS: You should really get a hold of using Blender if you’re going to be making stuff like that often.
Yeah, It’s still erroring like that :-/
Hey,
this is happening because u are trying to union 2 different texture’s and that will bug the union texture out. This is because unions only support 1 texture. Try to union only the DiamondPlate parts and it will be just fine.
In the Image below u can see the Neon texture.
Oh and u know that unions are the number one lag producers? In the topics below you can find more about it.
I hope this helped u out.
Greetings,
DrSyntaxx
Try splitting the union into parts by negating it every stud for example.