<?php
function isValidColor( $inputColor) {
    $len = strlen( $inputColor);
	if( $len != 3  &&  $len != 6) return false; // только длиной 3 или 6
	for( $i=0; $i<$len; $i++) {
		$code = ord( substr($inputColor, $i, 1));
		if( $code >= 97  &&  $code <= 102) continue;
		if( $code >= 65  &&  $code <= 70) continue;
		if( $code >= 48  &&  $code <= 57) continue;
		return false;
	}
	
	return true;
}
	

$tests = array(
	'FFF'		=> true,
	'FA0123'	=> true,
	'бла'		=> false,
	'<script>'	=> false,
	'000000'	=> true,
);	

$allGood = true;
foreach( $tests AS $test => $expects) {
	$result = isValidColor( $test);
	if( $result === $expects) {
		echo '.';
	} else {
		printf("\nTest FAILED for \"%s\": expected %s, got %s\n", $test, $expects, $result);
		$allGood = false;
	}
}

if( $allGood) echo "\nAll tests passed!\n";