I would keep the neutral grayish color that you have. It looks like a new paint job and fairly well done already. Its also a very neutral color. It looks really good.

A home feels more spacious and restful when there is a fluid color decor scheme. This will not be achieved with high contrast accent walls. You can use slightly darker toned paint colors to play with visual depth, like painting a wide shallow bedroom wall a deeper color -will make the room seem bigger by causing the wall to recede.

When we shopped for homes, we noticed a lot of folks think a wall color "makes the room" it does not. A painted wall does not equate to nice design, and same goes for a "bed in the bag" ensemble. Painted walls should be a background for your furnishings, not the focus.

What furniture are you bringing in to the space?

(Replace the builder boob lights in the hallway)

As far as deciding on decor..... Select both of your favorite colors and their different shades. Mix them with neutral toned large pieces, . Disregard the trend colors you see in stores. Next year there will be different colors, And the yesr after that.

Sherwin WIliams has some nice color palette collections. They take the guess work out of trying to figure out flow and making sure undertones don't clash. You can also use the color palette to find accessories and occasional pieces that will work with your paint color to add color and personality to your space. I've been partial to the coastal cool collection for years.

https://www.hgtvhomebysherwinwilliams.com/color-collection

Their website is a mess, to find the collections click the vertical black bar on the right side of the page.

Stick to one color in the main areas, including upstairs hallways. I think a warm white would look much better with your carpet and hardwoods than the current gray. You can paint your bedrooms or other rooms with some separation in coordinating light colors to have some differentiation, but you can decide on that later. It's a good suggestion to look at paint companies coordinated colors to help you.

d-gw. Provided a wonderful example for my comment above. If generally speaking, one of you likes blue, and one prefers green. You don't have to love the brighter or muted colors in palette. Notice the neutrals within that coordinated palette and you can have some degree of certainty that your green and blue wall art and other accents will look great with such a backdrop.