I think yellow would work especially with the colors you already have inside...you can always add a touch of yellow inside your home, even if it's something as simple as yellow flowers!

Yes, or a yellow vase somewhere near the front door. Good idea.

Your front door should be the focal point, so go with the yellow!

You can use a completely different color, as long as it won't look too tacky against your current scheme.

In your case, does not matter since yellow works fine with the greys and house theme