Allow for both hexcolors with alpha as shorthand

Author: vmcj

webapp/src/Entity/ContestProblem.php

@@ -261,7 +261,7 @@ public function validate(ExecutionContextInterface $context): void
     {
         if ($this->getColor() && Utils::convertToHex($this->getColor()) === null) {
             $context
-                ->buildViolation('This is not a valid color')
+                ->buildViolation('This is not a valid color.')
                 ->atPath('color')
                 ->addViolation();
         }

webapp/src/Utils/Utils.php

@@ -290,7 +290,10 @@ public static function timeStringDiff(string $time1, string $time2): string
      */
     public static function convertToHex(string $color): ?string
     {
-        if (preg_match('/^#[[:xdigit:]]{3,6}$/', $color)) {
+        if (preg_match('/^#[[:xdigit:]]{3,4}$/', $color) ||
+            preg_match('/^#[[:xdigit:]]{6}$/',   $color) ||
+            preg_match('/^#[[:xdigit:]]{8}$/',   $color)
+        ) {
             return $color;
         }