You can't define and describe a paint color (any color) using the theory that colors have undertones. Undertones in the common lexicon are just somebody guessing about what they think a color looks like.

Undertones are a subjective judgment made under unspecified, random light sources. Which means it's useless information beyond the individual making the judgment in their specific context.

Your situation is the perfect example of why the theory of "undertones" is irrevocably and fundamentally flawed.

Simply White doesn't have "yellow undertones". Nor does it have "grey undertones".

Simply White belongs to the yellow hue family. Nothing is hidden or under anything.

We measure grayness and colorfulness in terms of Chroma. How much Chroma a color of white has tells us how close to a true neutral white it is or how colorful it is.

Simply White has a Chroma value of 0.50. That's a healthy bump of chromaticity - which means it's far (ish) away from a true neutral white. Which is exactly why you're perceiving Simply White as NOT being a neutral, clean white.

You're describing that bump of Chroma, that amount of colorfulness, as grayness which is typical for colors of white from this yellow hue family neighborhood located over near the green-yellow hue family. The greenish overtones read as gray and certain qualities of light can emphasize that sense of grayness.

Simply White in context of its hue family.

In my experience one of two things could be happening in your room.

1. White is a highly reflective color, it reflects light and therefore reflects colors that are surrounding it. The denim blue is a deep grayish blue and when the color reflects off the white the white appears more gray.

2.. (IMO This is the more likely candidate) Your room lacks light. The eye needs light to percieve color and the less light we have the grayer/duller colors appear. For rooms without much light or lots of shadows we need more saturated color.