Reject multiple leading '#' in Color::from_hex

Found while testing crates/color/src/lib.rs:125

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/color/src/lib.rs

@@ -122,7 +122,8 @@ impl Color {
     /// Parse hex the way the colour field does: `#` optional, 1, 2, 3, 4, 6 or 8 digits
     /// (`F` → `FFFFFF`, `AB` → `ABABAB`, `ABC` → `AABBCC`).
     pub fn from_hex(s: &str) -> Option<Self> {
-        let s = s.trim().trim_start_matches('#');
+        let s = s.trim();
+        let s = s.strip_prefix('#').unwrap_or(s);
         if !s.chars().all(|c| c.is_ascii_hexdigit()) {
             return None;
         }

crates/color/src/tests.rs

@@ -114,3 +114,12 @@ fn infinite_channels_clamp() {
     let c = Color::new(f32::INFINITY, f32::NEG_INFINITY, f32::NAN, f32::INFINITY);
     assert_eq!((c.r, c.g, c.b, c.a), (1.0, 0.0, 0.0, 1.0));
 }
+
+#[test]
+fn hex_allows_at_most_one_leading_hash() {
+    assert!(Color::from_hex("##ff0000").is_none());
+    assert!(Color::from_hex("###fff").is_none());
+    assert!(Color::from_hex_or_name("##ff0000").is_none());
+    assert_eq!(Color::from_hex("#ff0000").unwrap().to_hex(), "FF0000");
+    assert_eq!(Color::from_hex("ff0000").unwrap().to_hex(), "FF0000");
+}