Can I mix orange / brown internally with black grey.

Orange is cheaper but aware that I can't use outside above ground due to UV but if not seen in a cupboard inside house.. Can I ?

Thanks

 

Yes you can use orange underground pipe internally but I would only use it for below ground, or below ground floor.
It would look very diy to mix it and remember most orange underground pipe cannot be solvent welded and could cause a problem in the future if an alteration or add on was needed - like a strap boss.