I have a very old piano. A few years ago, one of its bass strings broke. I removed that broken string and kept playing almost everyday for a few years. Last week my technician installed a new string made by its original producer for exactly the same type of piano, but it sounds quite differently. Why does the tone color change if the string is replaced by a new one? Is there any way to fix this problem?
Guitarists are accustomed to the fact that new strings have a different sound, I would say brighter in a sense. As they have not been used (yet!), their vibrating properties are different, no oxidation and so on…
What probably disturbs you is that it does not have the same color as the surrounding strings, creating a contrast.
If you are playing enough, the string will age and get its color closer to its neighbors which are old already. Another solution would be the wire cutter, but that would be a pity…
Like Tom says, a new string, even made by the same manufacturer will sound much different, most likely brighter than strings that have been on there for years. I suggest talking to your tuner about maybe softening the felt on the hammer of the new string, a process called “voicing”. This might help make the sound more consistent between the new and old strings.
The other answers are useful, but I think they missed the point.
Each piano key strikes 2 or 3 (depending on the pitch in question and the piano's design) strings. These strings are deliberately detuned just a hair so as to increase cross-resonance and enhance the tonality and the lifetime of the note.
When you were playing with the wrong number of strings under that key,you weren't hearing the design tonality of the instrument.