Obviously, once your tremolo is highly controlled, you can use a warm tone as well; in fact the gradual (or sudden) change of color is one of the most delightful devices you can apply in tremolo. And yet, if you train yourself to make a thinner sound your neutral tone for this technique, you will like the results.
(A quick word about thin tone in this context: I am referring to the sound produced when you play straight-across the nail at or near the sound hole. Not the sound we commonly refer to as ponticello or metallico. Therefore, it is the sound you get when modifying the nail angle near the sound hole, not that produced near the bridge.)
An oft-overlooked subtlety in the performance of tremolo works is the holding-over of melody notes to create a more legato line. This is done with such minute motor-control and subtle timing that it is, in fact, difficult. But it is also fair to say that it becomes intuitive with time and, in my own experience, seldom needs special attention once the technique (and the music) is well understood.
Take, for example, this passage from the Reverie-Nocturne of Regondi.
Here it is again, this time in the Gm section of Un Sueño en la Floresta, of Barrios:
Revisiting the opening of the Reverie-Nocturne, note the encircled bass notes. I would call these notes the bass line. All the other notes played by the thumb are essentially undifferentiated harmonic filler. This is so because the notes progress in a straight up-and-down arpeggio, simply outlining the chords:
In order to play this many parts at the same time convincingly, it is critical to understand what notes are most important, and what notes are mere filler. I like the effect of a strong bass, a very quiet "viola," and rather soft second violin and a quite strong melody, or first violin. This, of course, is "balance," the subject of several other posts on this blog. But somehow, doing it clearly and well while maintaining a consistent tremolo is challenging in a new way, and bears special attention. The most common effect I hear is that, if done "well," all the bass notes are soft but equal and the tremolo is loud. (More commonly, the bass notes are loud and the tremolo is inaudible, but this problem was addressed in my post Tremolo I). But a more musical reading of the texture would offer a more nuanced treatment of the various parts implied in the writing.
What we find in the course of the pieces, though, it that the textures change. When the player responds to these changes by playing the material with appropriate (evolving) balance, then the musical "truth" starts to really emerge. Take the tremolo opening of Un Sueño en la Floresta:
Here is another four-voice texture, with the static filler part now taken by the "second violin" and the internal moving part taken by the "viola." A bit of magic happens, though, in the fourth bar, when the two inner parts merge and present a gorgeous counter-melody while the tremolo remains still, listening. It is correct to "bring out" this counter melody, allowing it to shine, as long as, in the next measure, the original balance resumes.
Understanding how many voices are present at any one time, which ones are most prominent and which ones merely filler, and practicing for clarity of these shifting textures leads to more mature readings.
To summarize, The various challenges presented the player working on a tremolo piece are as follows:
1) Master basic bass-soft and tremolo-loud balance control
2) Learn to individually emphasize each finger for greater internal control
3) Work on gradually speeding up and slowing down your tremolo, for use in rubato phrasing
4) Consider the tone you use in playing tremolo, perhaps favoring a brighter sound than usual as neutral
5) Work on "true legato," connecting the tremolo notes over each new bass note
6) Evaluate the actual content of the accompaniment part and balance it accordingly.
There you go, no problem!! Have fun, and remember, everything you practice for the improvement of your tremolo applies DIRECTLY to other right-hand activities, so is a great investment. And never forget that non-guitarists find tremolo pieces especially appealing (and impressive) so the payoff for this work tends to be disproportionately high.