You will not be able to get replicate the Coldplay piano sound with a digital. A digital piano sound is too clean. When listening to Coldplay (I'm thinking particularly the Scientist) you can tell that it is a very small piano that has been recorded. It also sounds fairly percussive, but has been masked by reverb. Digital pianos, always try to simiulate large pianos, in a perfect environment. Thats how they sample them, big grands in acoustically perfect environments. And the sounds that they create for small pianos, sound like garbage.
The other thing with Coldplay songs, is that the piano is actually not in tune. If you play along with the scientist, your piano will sound awful. I haven't yet figured out if it was tuned with different octave stretching, or if they manipulated the temperment. That gives the song character, and makes it sound fuller overall, because it will fill up more frequencies around the fundamental, especially with the harmonics. Some people use the same technique with acoustic guitar.
If you want to try and make a digital sound more acoustic, or give it some life. Record a digital, put one of your speakers (it better be a good one) and place it at the top of an acoustic piano with the lid open. Face the speaker down into the piano, and mic it. The are a lot of variables as far as where you would like to put the mic. I've used this technique a few times with okay results. Took a lot of trial and error to get mic and speaker placement that I liked. I'll try and find some clips of it, and see if I can post them.
