Change triangle color on mesh

When you create the mesh also create an array of Color[] with the same number of values as vertices. e.g.

var colours = new Color[numVerticies];

Then for each colour in the array where the array index matches the vertex array index , set the colour.

If bottom left (BL) is index 0, top left (TL) = 1, top right (TR) =2, bottom right(BR) = 3 (in both vertex and color arrays)

colours[0] = new Color(0,0.1); // Note values are 0..1 not 0..255 colours[2] = new Color(0,0.1); colours[3] = new Color(0,0.1);

then set them on the mesh mesh.colors = colours;

HOWEVER - because both triangles share two vertex in common (BL & TR) those vertexes will will share the same color, and the top left triangle will have a blended colour. To correct this you need both triangles to each have three vertexes then you can have the BL & TR vertex with a separate colour.

In this case array size will be 6

You could create a second plane in the same spot and tilt both slightly so half of one is triangle 1 and half of the other is triangle 2.