When it comes to HEX values, possible values are A-F and 0-9, so it's proberly better to use 1 array, and grab a value this.
<?php
// REF: http://uk.php.net/array_rand
$hex_array = array(
'a','b','c','d','e','f',
'0','1','2','3','4','5','6','7','8','9'
);
echo '#'.
$hex_array[array_rand($hex_array)].$hex_array[array_rand($hex_array)].$hex_array[array_rand($hex_array)].
$hex_array[array_rand($hex_array)].$hex_array[array_rand($hex_array)].$hex_array[array_rand($hex_array)];
?>
Would output...
#e6aca7
#2bdfd2
and so on....
Of if you want 3 character values then change the echo for...
echo '#'.$hex_array[array_rand($hex_array)].$hex_array[array_rand($hex_array)].$hex_array[array_rand($hex_array)];
?>
If you specifically want the values you postsed... then just changes what's in the array and the echo..
Like this...
<?php
$hex_array = array(
'#F90', '#0F0', '#F06', '#0FF'
);
echo $hex_array[array_rand($hex_array)];
?>
Hope that helps.. :)