Probleme with imageColorAllocate function

Hi there, I'm new in this list, so hello everyone. I would like to know if somebody have already got the following probleme with the imageColorAllocate function: I use it in a loop and the function return the right color id but only in the beginning. After some loop the function return always -1. Here the code: $colors = array ( "yellow" => array (255, 255, 0), "blue" => array (51, 102, 153) ) ; for ($j=0; $j<20; $j++) { ... list($red, $green, $blue) = $colors[$info[3]]; $i_color = imageColorAllocate($small_img_id, $red, $green, $blue) ; ... } info: $info[3] is switching between "yellow" and "blue". If somebody knows why after one or two loop $i_color is always equal to -1, I'm pretty interested by his solution ! Thanks Pascal