Blending textures on a random terrain?

Okay so I have created a random terrain using perlin noise 3D. What i have done is make it so that if the terrain is above a certain height i used setpixel to change the color, creating green grey and blue areas. The problem is i want to make it so that instead on colors it is a texture. I Carnt make a texture as the terrain is generated at runtime so is there a way to set induvidual materials instead of using setPixel? Thanks :D Oh and btw im using c#