Trail renderer change color without changing all previous color

Hi, I'm using trail renderer to trace the position of the user and prints it on minimap. The material of blue color is being used. I want to change the color if the user stays at one moment like 10 or 20 seconds. So I changed the color of material to red through the script when it is needed. So trace now turns to red.

// code...

trailRenederer.material = redMaterial;

// code..

But the problem is that the older trace with blue color does not remain as blue. It turns to red as well. I want them to be remained as blue. How or what can I do?