High-quality exterior house paint can be pricey, so you definitely want to get your colour choices right the first time to avoid wasting it. Tying in your exterior walls with the colour of your roof is necessary if you want to avoid a fractured or awkward appearance.
One way to tie in your exterior walls with a grey roof is to paint them a different shade of grey. If your roof is dark grey, you can paint the walls a light grey and they will complement the roof, without creating too monotone an effect. Conversely, if the roof is light grey, dark grey walls can accomplish the same thing. Be sure to test the paint look before coating your house with it. Remember that a colour can look different on a paint sample than it does on a broad surface such as an exterior house wall.
Consider contrasting the walls with the subtle grey roof by painting them in a bright colour. Be careful with bright colours, as indulging in a straight primary colour like red, blue or yellow, or experimenting with neon colours, can be overwhelming when covering an entire house.
A somewhat more muted shade of red or yellow can create a bright house that will be balanced by the grey roof, creating a dramatic tension that can be pleasing to the eye. If you have access to an architectural design program on your computer, you may have features that allow you to create designs and fill them in with different colours. This can give you some idea of what your house will look like before you begin painting.
The greatest advantage of white paint is that, like a white shirt, it doesn't really clash with anything. A white house with a grey roof can work well, especially if it is an older home with a traditional profile. Many older homes, particularly in New England, are sided in white clapboards. Painting your walls white, and then painting the trim in a colour that matches the grey of your roof, can bring the entire house together and make it look balanced and attractive.