Texturally speaking, a tear would only alter the light reflection characteristics of the skin underneath, not the color. You could easily use photoshop's layer effects for this.
1. add a new layer for the tear streak
2. create a bezier path for where the streak should go
3. stroke the path with a dark, solid brush with the width of the tear.
4. set the layer blend to "screen" so the black becomes transparent
5. set your layer effect to "bevel/emboss" and experiment with the reflections.
On a water drop there's also the shadow of the water so you might darken the underlying skin a little where the streak goes.
You could add a second layer with just an oval for the teardrop, repeat the above, and spherize the underlay to simulate refraction.
Water droplets act as lenses so a little highlight on the skin surface under the tear would enhance the 3-d look.