Use WHITE and BLACK constants where appropriate

rtchallenge/examples/eoc6.rs

@@ -7,6 +7,7 @@ use rtchallenge::{
     rays::Ray,
     spheres::{intersect, Sphere},
     tuples::{Color, Tuple},
+    WHITE,
 };
 
 fn main() -> Result<()> {
@@ -28,7 +29,7 @@ fn main() -> Result<()> {
         ..Material::default()
     };
     let light_position = Tuple::point(-10., 10., -10.);
-    let light_color = Color::new(1., 1., 1.);
+    let light_color = WHITE;
     let light = PointLight::new(light_position, light_color);
 
     for y in 0..h {