How do I open up a window I have closed in blender? I opened up a new viewport window to move lights, adjust camera, etc which was just grayscale. When I meant to close the new window, I accidentally closed the window that had my materials rendered on the screen so there is no color being rendered. What do I do to get the colors back?
This image should have pink, brown, and blue...

This image should have pink, brown, and blue but it's just grayscale
In the top left of any viewport window you'll find this button:

You can turn any window into any other type of window from here, and completely customize your workspace.

In your case, you were in the 3d viewport, but you were in either rendered mode or material preview, and now you're in solid mode.  You can switch between these by pressing Z while your mouse cursor is inside the viewport, or using these buttons in the top right.

Last, if you don't know what you just closed, you can create a new workspace of the type you were working in, here.

You can delete the old workspace and move on like nothing happened.