Unioned have a grey color on the inside of them

[Derpboxproductions (Autumn)]

For some reason when im trying to union these 2 parts, the inside of the part is a greyish color

[fun1892]

Try going into the properties of the part and choosing to use part color
if you have not tried this already

[Crazedbrick1]

You must change the color of the part that was turned negative into red.