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: 
You could try the imagefilter function <http://lv.php.net/imagefilter> - but that will not give your direct access to replace one color with another, just changing the r/g/b components.

A very low level solution could be implemented using imagesetpixel <http://nl2.php.net/imagesetpixel> to set the new pixel values.
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...