Go Coral.

Red. There is something about a red door, very welcoming and warm.

Yes! A warm "bricky" red. While your other colors are cheerful, they are all cool, blue-based, even the white, so something with a yellower base would be great. You'll get it.

Adorable house and the yellow door is beautiful! The shutter colors are beautiful too! I do agree, something is not 100% right! or maybe just something is lacking, although all three colors work so well together. It seems that yellow doors work better on gray homes, maybe it just "pops" more against a darker color, so maybe adding some bright yellow flowers/shrubs around your front porch would draw more attention to that beautiful yellow door and blend it all together more. Maybe some large terra-cotta pots with some yellows/oranges/red flowers up the sides of your steps would fill in the blank space and make the front area more of a focal point. :) Here are some pics of different colors for your front door just for ideas...all are from Sherwin Williams :) Also, some pics of how adding flower pots can draw attention to a front door and tie colors together :) Hope these help! Have fun!

Here's a couple more pictures of plants on steps I found :)

I like the yellow you have. A buttery one would work, or one with a little ochre. You could check the Historic colors.