The particular ’color’ or timbre that the glottal tone will have when it leaves the speaker depends on the shape of the speech tract. This shape can be altered quite a bit, mainly by moving the tongue and lower jaw into different positions and by rounding or spreading the lips. Since every shape will give its own characteristic resonance to the air contained in it, we will generally be able to produce as many different vowel sounds as we can give different shapes to the speech tract.
Try to ‘mouth’ the vowels of Dutch without making any sound and notice how your tongue and lips move to different positions for them. Since our speech tract will always be with us during our speaking lives, resonance will always be a contributing factor to the quality of the sounds we produce, even of voiceless sounds. This is easy enough to demonstrate: say [ss] with spread lips and then round them, and notice how the sound produced becomes duller.