There are many sites dealing with the guides how to change the colours of your HUD in the game. It became widely known at the beginning of December 2012 (here is a thread from the official forum). Let's cover them and bring you the best of them and give you also some good advices.
Let's start with the basics.
To change your HUD colours (and other options too) you must go to your Elite Dangerous game folder and to the directory: Elite Dangerous > Products > FORC-FDEV-D-1002
There is a file called GraphicConfiguration.xml. You can manually change the settings inside in notepad, text editor, Total Commander or whatever. And the lines responsible for colours are these (you can find them by CTRL + F and write "<GUIColour>". Make a backup of the file to be sure that you can get it back to the default state.
The numbers responsible for the colours are in bold and in by default it's like it's below:
<MatrixRed> 1, 0, 0 </MatrixRed>
<MatrixGreen> 0, 1, 0 </MatrixGreen>
<MatrixBlue> 0, 0, 1 </MatrixBlue>
Where to find HUD pictures, examples and guides?
1. You can find quite a lot of samples with pictures at reddit page. It' a little bit chaotic and the most important things there are the options with pictures.
2. Another guide covering the same facts but with another settings with pictures to show is incgamers site.
3. But I highly recommend to visit this page where you can adjust all the numbers by sliders and it's very user friendly because you can see immediately the results at the picture above sliders. And then you just copy the numbers and write them to the xml file. And what's more you can make a link to your favourite HUD and show it to your friends. Nice job Kimmo Kulovesi!
4. Of course there are also video guides which use the Kimmo's site. But if you need help and how to do everything then it's made for you.
As you can see it's nothing complicated but the experience of flying in YOUR ship is much better because you have changed the look of the cockpit and can say to yourself: "Yes, this is my f...ing awesome Anaconda!" And if you add some external paintings... :-)