I do not understand what the tone is you want. It is gray with a tinge of blue making it lighter will not change that , why not just choose the one down from it on the card

Brewster Gray is very dark with definite blue undertones, so as @Patricia Colwell Consulting says, lightening it will not change the blue undertone.

A suggestion for a gray that is some what brighter: Cloudy Sky. It has a blue/green undertone should work with the different natural light coming in your windows (you don't say what direction the room is facing).

I deifnitnely want a blue gray. Wanting to use it on the extrior of our house for the shutters and front door. The body of the house will be BM Shale and trim is White Dove.

Have you tested the paint out on the exterior of your house? Paint colors are quite a bit lighter when they are on the exterior = it may be lighter and show more of the blue.

Shale and Brewster Gray look like a very nice combination. However, you need to view the colors where they will be seen. Comparing the samples to online color is not telling you anything; online images are notoriously unreliable at rendering the actual color.

Take the sample outside and view it in natural light, preferably after the house is painted, or if that's not possible, compare the samples for the two colors together. If it still looks too dark and gray, you need to find a lighter color with more blue in it. You might try Water's Edge (1635), Santorini Blue (1634), Comet (1628), Manor Blue (1627), Little Falls (1621), Blue Heather (1620).

Diluting it 25% with white will make it lighter, but probably won't affect the blue/gray balance. You might try Little Falls or Silver Mist. If you click on a color on the BM site, they will show you lighter/darker and similar colors.

For example, with Brewster, they offer Adiago as lighter, Templeton as darker. Similar colors include Shadow Gray, which is a tad bluer and lighter, Gull Wing gray which is lighter. You can keep clicking until you come across a color you love.

Benjamin Moore Brewster Gray is a shade of cyan. If you go 25% lighter, it is a medium light shade of cian Marina Gray BM.

https://www.decorpad.com/photo.htm?photoId=111296

Marina Gray BM bathroom.

The OP wanted something a bit lighter and more blue. Marina Gray is very gray and I don't think it would look very good with Shale--not enough contrast and too gray.

Shale works much better with a darker blue like Water's Edge.

Thank you everyone for input. This is a new construction home. So I’m having to make tons of decisions, and having occasional decision fatigue snd questioning my choices.

Cutting the formula would make the color strength weaker. Because of the DNA of the colorants, you would have to add a hefty amount of white paint to move its appearance..

Color online is not accurate and the kind of blue gray color you’re describing falls into a pocket where there is an even greater inaccuracy online.

Even though there are a lot of decisions to make, it’s a really good idea to make the time to go to a paint store and get chips and paint samples to test the colors properly

And don’t let anybody rush you in that process. If it takes you a couple more days to test and find the right colors , they can wait.