This is a picture of my staircase after i made it a union. For some reason random parts turn dark grey even though i have no dark grey parts selected. I know this is a small post but any help is appreciated!
UPDATE:
Just discovered that this is only on the top side of some parts.
You should check the “Properties” tab and see if you have “UsePartColor” enabled.
Okay, I would recommend having the yellow stripes separate from the stairs as that way you can change the colour of the stairs without having the yellow stripes being changed too.
I’ve done that but i still have the same issue of different coloured bits. I have UsePartColor enabled and have fiddled with the colours but no change.
I have tried recreating your stairs and it seems I have the same issue with the union and it changing the colour even though that colour wasn’t present. I think it might an issue either with the lighting or just the union itself.
However, if you pull the part down a bit it seems to slightly fix that issue as you can see here
Hmmm… this never happened to me, but maybe it really is something with the lighting maybe try using diffrent technology such as future!
I’ve tried that and it works fine, thanks for helping me! I’ve been having this problem for ages lol and at least i have a solution now 
