Move color and rectangle structs to separate module

src/color.rs

@@ -0,0 +1,22 @@
+/// Color struct.
+///
+/// Represents a color with RGB values from 0 to 255.
+impl Color {
+
+    /// Constructor.
+    ///
+    /// The Red, Green and Blue values must be between 0 and 255.
+    pub fn from(r: u8, g: u8, b: u8) -> Color {
+        Color {
+            r,
+            g,
+            b,
+        }
+    }
+
+    /// Get a hexadecimal representation of the color,
+    /// such as `FFFFFF` for white and `FF0000` for red.
+    pub fn as_hex(&self) -> String {
+        format!("{:02X}{:02X}{:02X}", self.r, self.g, self.b)
+    }
+}