diff --git a/crates/color/src/lib.rs b/crates/color/src/lib.rs
index ce3340162..0c271a3a8 100644
--- a/crates/color/src/lib.rs
+++ b/crates/color/src/lib.rs
@@ -296,7 +296,11 @@ pub fn hsl_to_rgb(h: f32, s: f32, l: f32) -> [f32; 3] {
 
 /// Parse `#rrggbb` / `#rrggbbaa` into 0..1 floats.
 pub fn parse_hex(s: &str) -> Option<[f32; 4]> {
-    let s = s.trim().trim_start_matches('#');
+    let s = s.trim();
+    let s = s.strip_prefix('#').unwrap_or(s);
+    if !s.bytes().all(|b| b.is_ascii_hexdigit()) {
+        return None;
+    }
     let b = |i: usize| u8::from_str_radix(s.get(i..i + 2)?, 16).ok().map(|v| v as f32 / 255.0);
     match s.len() {
         6 => Some([b(0)?, b(2)?, b(4)?, 1.0]),
@@ -318,6 +322,16 @@ pub fn to_hex(c: [f32; 4]) -> String {
 mod tests {
     use super::*;
 
+    #[test]
+    fn parse_hex_rejects_signs() {
+        assert_eq!(parse_hex("#+f+f+f"), None);
+        assert_eq!(parse_hex("#-f-f-f"), None);
+        assert_eq!(parse_hex("##0f0f0f"), None);
+        assert_eq!(parse_hex("#0f0f0"), None);
+        assert!(parse_hex("#0f0f0f").is_some());
+        assert!(parse_hex("0f0f0f80").is_some());
+    }
+
     #[test]
     fn ycbcr_roundtrip() {
         for m in [Matrix::Bt601, Matrix::Bt709, Matrix::Bt2020Ncl] {
diff --git a/crates/engine/src/captions.rs b/crates/engine/src/captions.rs
index d048a5677..02bddd456 100644
--- a/crates/engine/src/captions.rs
+++ b/crates/engine/src/captions.rs
@@ -82,7 +82,11 @@ fn parse_color(v: &Value) -> Option<[u8; 4]> {
             _ => None,
         };
     }
-    let h = v.as_str()?.trim_start_matches('#');
+    let h = v.as_str()?;
+    let h = h.strip_prefix('#').unwrap_or(h);
+    if !h.bytes().all(|b| b.is_ascii_hexdigit()) {
+        return None;
+    }
     let b = |i: usize| u8::from_str_radix(h.get(i..i + 2)?, 16).ok();
     match h.len() {
         6 => Some([b(0)?, b(2)?, b(4)?, 255]),