Your two photos prove your argument and did prove theirs!
The base unit has, what is called in the trade, a decorative end panel.
The tall unit does not and therefore it is wholly unacceptable.
You should ask the housebuilder's directors if they would accept this in their own house?
Yes the standard carcass colour is cream/magnolia but so is white!
I have white inside my cupboards but the doors are oak and yesm,the panel on my tall fridge unit is matching wood effect - not white!
Stick to your guns on this.
This is a pity, as it does look like they are trying to protect the floor and worktops, which demonstrates they care more than most of the plc housebuilders do on their sites!