While I don't understand how a thinner product (assuming it is solid/opaque and not somehow translucent/transparent) would be a different shade than the thick version, the sales rep has obviously run into this because he is warning you about it.

What does make sense, however, is that the product (any product) looks different because of the change of plane. Light will hit the backsplash from a different angle than it will hit the countertop. Depending on the material, that change in light could be noticable.

Please be aware than I have no experience with the specific products you have mentioned.

You’re struggling, imagine how hard this is for us! Please provide images of your materials and space. Thanks.

This is a design of the kitchen. The dark brown/walnut cabinets are actually white oak

To many different things already. If this has a hope of coming together in a cohesive design, one of the cabinet choices needs to be repeated as the counter. That means plain white that 100% matches the cabinets. Or wood that matches the cabinets. No black. There’s too much nonsensical plop plop use of black already, and it probably should just go away already.

IMO 3 cabinet colors is for sure one to many I do not agree the counter has to "match " The cabinets but we have way too little info to have a clue and the pic you posted is not helpful at all since we cannot even see the design of the space

I don’t think your kitchen designer really gets modern design, or proper space planning, with the right clearances. It’s very disjointed. The black looks interjected because someone felt the need to ”add interest” because they couldn’t leave modern simplicity well enough alone.

No quartz splash, because it scorches.

Again no help from the OP other than posting a pic that is in no way helpful regarding their actual choice s and space .