Painting rooms partially, how to get good edges
Professional painters don't use tape. 

They "cut in" the wall by using an angled brush in the corner. Usually a 2" (50mm to you) sash brush. This requires a fair bit of practice and a steady hand but once you are accomplished at it, it is much faster and even looks better then taping off, and even beats hi-tech painter's tape. 

Then, they roll the wall right up close to the corner and you can't really tell they brushed the corners.

If you paint break to break (corner to corner or floor to ceiling) then a very small difference in shade is very difficult to see because of lighting. Additionally, you don't need to try to match the stipple of the of the existing paint.

Always save leftover paint in a stable temperature that doesn't freeze and you will be able to touch up nicks and scratches for years before having to change colors.

Good luck!