While I prefer to paint rooms completely, including the ceiling, I sometimes need to paint a single wall, or even a limited area of one wall.
For example, the kids may have drawn directly on the wall, or in the entrance area just one area of the wall is very smudged etc., and there may be a lot of furniture on the "good" walls which would just be not worthwhile to move around.
That said, what are good techniques to avoid ugly edges where your new paint meets the old? For example, at wall edges, would you rather stick on your masking tape on the wall that you are painting, or on the adjacent wall? Or would you avoid hard edges where new paint meets old, but somehow smooth the paint into the old areas? I saw an old professional painter do that once and it turned out fine, but I really have no idea how I would do something like that consistently.
I am mostly talking about the commonplace western europe white interior paints, not latex based paints.
Best Answer
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!