It works!

src/main.rs

@@ -9,11 +9,11 @@ use ansi_term::Colour::RGB;
 fn main() {
     let stdin = io::stdin();
 
-    for line in stdin.lock().lines() {
+    for (n, line) in stdin.lock().lines().enumerate() {
         let line = line.unwrap();
 
-        for (i, c) in line.chars().enumerate() {
-            let (red, green, blue) = rainbow(0.1, i);
+        for (o, c) in line.chars().enumerate() {
+            let (red, green, blue) = rainbow(0.1, n + o);
 
             print!("{}", RGB(red, green, blue).paint(c.to_string()));
         }