Is there a reason why the colors is interpolated on the CPU? i think it would be more flexible if the colors where just passed to the Particle shader together with a normalized float representing where in the lifecycle the particle is. Then the interpolation could just be done in the shader, preferably in the vertex shader.
If the lifecycle float is passed to the shader it would be easier to implement a customized particle shader. One could for example use a 1D-texture to create gradient effect. Ofcourse one can just abuse the StartColor and EndColor values and let the color channels represent some other attribute… But I think thats a ugly approach. 
Because we need the particles to work in OpenGL1 mode where shaders cannot be used