Trail renderer colors/materials relationship

Is there a simple relationship between the materials and the colors? I find the relationship confusing; for example I created a material which is all white (at full opacity) and then selected a series of green shading to yellow with decreasing opacity - this resulted in a GREY trail! I omitted the material entirely in an attempt to emulate the behavior of a particle system but that resulted in a pink trail no matter what the colors.

"Pink" is magenta, and means your material is missing or otherwise broken, e.g. if your shader has errors. The colors are stored in the vertices.