I agree with Todd's answer; it's mostly a matter of personal preference. However, it should be noted that a lot of the sound depends both on the style of the piano and the tuning. The style may not be an adjustable factor depending on your circumstances.
An new upright piano will generally be more mellow than a grand, for example. And like Todd says, they can get brighter with aging. With uprights you will also be more likely to find that the soundboard softens after many years which will dull the tone.
Tuning an instrument a little higher will also give a brighter sound, as was seen historically with pitch inflation. With a piano with multiple strings for each key, you can also offset the strings very slightly from each other to give the effect of a tack or ragtime piano (which is sort of bright and muddied at the same time).
Given the great electronic pianos we now have with key weighting and touch sensitivity, you might want to consider one of those — most come with several different timbre presets so you can select a brighter or mellower sound at will. I really enjoy having the ability to select a harpsichord sound for certain pieces, myself.