RGB
RGB is an additive color model in which a color is produced by combining three channels of light: red, green, and blue. Each channel usually holds an integer between 0 and 255, giving 16,777,216 possible colors.
PHP’s gd extension works natively with RGB: imagecolorallocate() takes three integers, one per channel, and returns a color identifier that can then be used to draw on an image. RGB values are also commonly written in hexadecimal notation, #RRGGBB, as used in CSS and HTML.
RGBA extends the model with a fourth, alpha channel, controlling transparency.
<?php
$image = imagecreatetruecolor(100, 100);
$red = imagecolorallocate($image, 255, 0, 0);
imagefill($image, 0, 0, $red);
?>