Improve color detection across platforms

Fixes #882

src/style.rs

@@ -161,9 +161,24 @@ pub fn style<D: Display>(val: D) -> StyledContent<D> {
 ///
 /// This does not always provide a good result.
 pub fn available_color_count() -> u16 {
-    env::var("TERM")
-        .map(|x| if x.contains("256color") { 256 } else { 8 })
-        .unwrap_or(8)
+    #[cfg(windows)]
+    {
+        // Check if we're running in a pseudo TTY, which supports true color.
+        // Fall back to env vars otherwise for other terminals on Windows.
+        if crate::ansi_support::supports_ansi() {
+            return u16::MAX;
+        }
+    }
+
+    const DEFAULT: u16 = 8;
+    env::var("COLORTERM")
+        .or_else(|_| env::var("TERM"))
+        .map(|x| match x {
+            _ if x.contains("24bit") || x.contains("truecolor") => u16::MAX,
+            _ if x.contains("256") => 256,
+            _ => DEFAULT,
+        })
+        .unwrap_or(DEFAULT)
 }
 
 /// Forces colored output on or off globally, overriding NO_COLOR.