Прошло 2 года.

lib/epdiy/examples/screen_diag/main/commands.c

@@ -0,0 +1,17 @@
+#include "commands.h"
+
+bool validate_color(uint8_t* inout_color, struct arg_int* arg) {
+    int user_color = arg->count != 0 ? arg->ival[0] : *inout_color;
+    if (user_color < 0 || user_color > 0xFF) {
+        printf(
+            "Invalid color %d (0x%02x): Must be in range 0x00 to 0xFF.\r\n",
+            user_color,
+            (uint8_t)user_color
+        );
+        return false;
+    }
+
+    *inout_color = (uint8_t)user_color;
+
+    return true;
+}