Good clarinetists will often speak of changing the tone color to highlight a particular harmony. Unfortunately, it is rare to hear someone speak lucidly on the precise methods for producing various colors. The clarinet does not have the wealth of possibilities that the violin has. The violinist can alter the speed of the bow, the angle of the bow, the pressure of the bow, and the location of the bow in relation to the fingerboard and the bridge. Our situation as clarinetists is more akin to that of pianists: often we must imagine that we are doing something different in order to change the tone color. However, there are a few practical things we can experiment with.
An essential ingredient in accessing multiple tone colors is a light reed. It does not need to be a 1 or 2 reed, but it should be on the lighter side of the scale relative to the strength of reeds we use on a daily basis. This is because a light reed is more sensitive to changes in air pressure and air direction.
I have identified three general methods for achieving different tones colors on the clarinet.
We can give the impression that we have changed tone colors by altering the dynamic. The change in dynamic needs not be very large, but if it is timed well to coincide with a particular harmony, it can give the audience the effect of a spine-tingling change in colors.
Air pressure/air direction/shape of oral cavity
There are several variables at play here. Concerning air direction, you can direct the air right at the reed or you can try to direct the air towards the roof of the mouth before having it come downwards into the mouthpiece. These two strategies produce different sounds.
You can also modify the space inside your mouth while you are playing. Your oral cavity can be open behind the seal of your embouchure and the mouthpiece, or it can be narrowed and focused. This affects the resonance of the tone.
You can also experiment with the air pressure. Less air pressure generally produces a less compact sound.
These variables can be combined in various ways. For example, using high pressure air and directing it right at the reed generally produces a harsh sound which is perhaps appropriate only in 20th century music, but high air pressure with an open oral cavity and non-direct air produces a rich and resonant sound.
Vibrato can be created with the lower lip or with the diaphragm. In addition, vibrato can be continuous or non-continuous. I add diaphragmatic vibrato only to some notes to add a certain expression to them. Often I add the vibrato spontaneously, but occasionally I plan to use vibrato on specific notes.
Playing with colors means rejecting clarinet orthodoxy to a certain extent. Many orchestral clarinetists tend to worship a particular sound. Any other sound is often deemed inferior. When you produce different colors, you are indeed producing different sounds, some of which may be less beautiful than others. But, I think that we should move away from an idealization of one sound towards thinking that there are many acceptable sounds, but that some are more appropriate in certain situations than others. The color of the sound should be chosen to express the meaning of the music as you perceive it.