Question: I got the image like this (it's a graph):

[![Gold Trade Graph](https://i.stack.imgur.com/vyayH.gif)](https://i.stack.imgur.com/vyayH.gif)  

(source: [kitconet.com](http://www.kitconet.com/charts/metals/platinum/tny_pt_en_caoz_2.gif)) 

I want to change the colours, so the white is black, the graph line is light blue, etc.. is it possible to achieve with GD and PHP?

Answer: 
IMG\_FILTER\_NEGATE: Reverses all colors of the image. 

<http://www.php.net/manual/en/function.imagefilter.php>

Could that be a solution?
I'm not aware of any ready-made functions. But I suppose you could go trough every pixel of the image and change it's colour...