Those of us living with dementia have big problems with both guessing distance/range and seeing color on same color.

THANK you for trying to find out the WHY of what is happening. !  
As Persons Living with Dementia (PLwD) our brain is dying.
We are in the various phases of brain-failure, and the brain processes visual data as well as every other sense. 

What our eyes see is interpreted by the brain,
and this is probably a brain-failure issue.

Your Loved One NEEDS to have the bathroom door a different color than walls; possible problem of distance perception.  It is surprising how many of us have nightmares of not being able to find the bathroom door.

He needs to have toilet and toilet seat a different color than the floor and walls; again a distance perception issue. Otherwise eventually he will not even be able to FIND the toilet, let alone lose his marksmanship.  

Same is true of plates on the table; very beneficial to have plate different color than the table (or tablecloth). And it helps to have trash bins a different color too.
Several folks have insisted that their loved one throws trash on the floor out of anger, and that is POSSIBLE, but unless the care-PARTNER is actively researching why we (as PLwD) do things, it is very easy to blame .

Personally i still have a bit of distance perception remaining, but it is going fast;
i dont realize i am setting my mug on the very edges of table (because placement of the “edge” is all wonky).
My “estimator” for everything from where the corner of the wall/door is, to time needed to dress for an event is broken (probably because requires CONTEXT of past experiences, and that is all going haywire).

Important to remember that your Loved One’s abilities are changing, and it feels like as soon as we get some strategies in place to help a specific symptom, then our brain deteriorates more and that strategy becomes less effective, and needs adjustment.

Husband says “New strategies need to be developed constantly; keep your strategies current, for which specific issues you are dealing with.”
Symptoms change, and those constant changes feel like walking a tightrope every single minute of every single day. 
i know it is exhausting for care partner, but it is also extremely exhausting for Person Living with Dementia (PLwD).