Hi, I like the overall concept of the darkish grey, it suits the home. I am a colour consultant and have been working with the Dulux range for 17 years and you are on the right track with the Guild Grey. The Dulux Signature will give you a dark blue house. Another option if Simone Weil which is just a bit lighter than the Guild Grey, if you find that one too dark. Sample pots and A4 brush outs onto white cardboard are the best way to decide, because computer programs give an overall impression of the "look" but are not accurate with colour choices, you can't beat the human eye. All the best, it's going to look great.

Can't go wrong with Cabots

personally I like a home that has a similar style on the inside as the outside, it just makes sense to me. In the past I thought building extensions should be similar in style as to the original, however after reading (and commenting on) Houzz posts I am now coming round to the idea of a modern extension on an older building. But having a kitchen and bathroom in different styles, that might just be to your taste and that is fine - there is nothing wrong with a building reflecting the personality of the owner!

grey and white always look good but Google "choosing colour schemes" for all the different paint companies services for matching colour schemes