I agree, when mixing classic with contemporary, it is best to have the accent chair be either modern or traditional. Here is a link to an article that has some great tips on mixing traditional with modern elements: http://cclifestylesbydesign.wordpress.com/2014/02/03/design-fusion-mixing-matching-without-clashing/
If you are located close by, I would be happy to offer you a complimentary in studio design consultation.

Thank you both of you!! I read the article, it has some great ideas. Im more inclined towards the modern look so an accent chair is a good idea, but what colour do you think would look nice with dark grey? Also does it have to be leather or i can have a fabric sofa with a fabric chair?

Also i would have loved to have a design consultation but i live in London, im really confused about what type of chair i should go for so i would appreciate if you could just give me some ideas? Thank you!!

The beautiful thing about grey is it goes with any colour under the sun. I'm a big believer that you should live with what you love, so a colour that you naturally gravitate towards would be best. Here are some modern styles from some of my suppliers that could work well with your sofa. There is no rule saying that you have to go leather, the biggest thing is to have a different texture, tone or colour.

Design is all about quality of life and I am happy to provide a little help when I can. If you are close by, I would be happy to book an in studio consultation with you to get you set in the right direction. We also offer edesign packages if you need more assistance.

I always mix...

I have 2 chocolate brown sofas in my great room similar to the sofa in the picture above. I decided to place 2 orange and cream armless chairs in the room.

Grey works with most colours as Chrissy stated above - just ensure that your tonal contrasts are balanced (as a tip I advise people to take a black and white photo of their room) you will automatically see where there are areas that are just too dark and need some lighter colours to "bring them up"

Some more ideas:

Thanks all of you for your suggestions, i really appreciate that. Also does this kind of tv unit go with this grey sofa, if not can i have some suggestions please! Im thinking of putting the tv unit where the piano is at the moment in the second photo? (All this stuff wouldn't be there when we move)Thank you!!