The tone quality you produce depends on your fingers and, perhaps more importantly, the shape of your fingernails. Tone production goes hand and hand with fingernail shape which is worthy of a complete, separate discussion. Establishing a good overall sound can take time and realising/understanding what it is comes with experience. This discussion assumes that you have nail shape under control and are already producing a good sound.
Players who know how to make a good natural classical guitar sound, can happily play every note with exactly the same sound or tone colour. In fact many players do this in order to ensure right hand finger security and control. Keeping your right hand in the same position reduces the risk of making mistakes such as plucking the wrong string. However, the disadvantage of relying on one sound is that after a while, the music becomes bland and uninteresting, lacking imagination.
Varying your tone colour can add spice (in other words interest) to a music performance and avoid audiences ‘switching off’ or their minds wandering. It is similar to dynamics or how loudly you play. If you played every moment of every piece at the same volume level, the performance would be very predictable, possibly boring, for the audience.
The challenging aspect of tonal variation is knowing exactly how much variation is enough. You have to be careful not to over-do it and to use tone colour variation sparingly but at just the right moment for the greatest effect. That takes experience and is to an extent a matter of personal taste but can also be specified by composers. Without any specific instructions from the composer and as a general rule of thumb, it is a good idea to start a piece with a natural sound and to wait for a new phrase or section of the music to adjust the sound. It is often very effective to change the sound when there is a key change, for example going from a major key to minor. You might switch from a normal sound to soft and sweet. One of the most challenging ways to use tonal variation, and that takes great coordination, is to play a melody with one tone colour and an accompaniment (chords for example) with another.
The three basic tone colours are ‘natural’ (normal sound), ‘dolce’ (sweet/soft) and ‘ponticello’ (brittle/bright). There are also various shades in between.
‘Natural’ sound can be achieved by plucking the strings near the sound hole, i.e. just below the edge of the sound hole, towards the bridge.
A softer, sweeter sound can be achieved playing over the sound hole nearer the fretboard. Playing over the 19th fret will produce a softer sound again. Some players go further still and pluck the strings over the 12th or 13th fret which is particularly effective on bass strings.
Ponticello is achieved by plucking close to the the guitar bridge. The nearer the bridge, the brittler the sound becomes. Brightness of sound is associated with playing nearer the bridge. Again there are varying degrees of brightness depending on where the fingers strike the strings.
To add further complication, there is another way to achieve tone colour variation. Advanced players can change their tone by making subtle changes to the angle that their fingers/fingernails strike the strings. That provides a way of changing the tone without moving the hand away from its usual or preferred position. For some players this is enough on its own however there are times when a more dramatic change is useful and calls for a complete change of hand position.
Of course, changing your right hand position comes with a certain degree of risk. You have to become confident about moving the hand while locating the correct strings so that the flow of music can continue unimpeded. It is not uncommon for players who take such risks to stumble as the classical guitar can be a very unforgiving instrument. It’s important to remember that there are times when the music is so complicated and fast that there simply isn’t enough time to vary tone by changing your right hand position. On the other hand, when the risk pays off the end result can be outstanding and really grab an audience’s attention.
Using tone colour variation correctly also relies on musical taste and understanding. If you are not sure what to do, it can be helpful to observe what accomplished, professional guitarists do. At the same time do be mindful that some players soften their right hand technique during recordings in order to produce an even, ‘microphone friendly’ sound. However, most players want their recordings to reasonably represent their concert sound and to retain their chosen tonal variations.