COLOUR
In order to change it you need and hex editor, I use Hex Workshop.
Select the colour you want to add and get its hexadecimal form, as seen on imagen1, and invert the fields this way: D8360A --> 0A36D8

Now open the xxcrt.o with the hex editor and locate the parts belonging to the colour (To make it easy and find them in a row i use a screen resolution of 1024x768 and i semi open the window). Imagen2 is the part of the outcourt colour and Imagen3 is the key court colour. Now replace the data that end with FF.


OEDIT
Oedit is very intuitive, you must practice with it, and if you think you get something, go and save it before you mess it up.
One thing you can't see as easy is the texture positioning, on Image4 I managed to give different colours to lines that at the first place had the same colour. Select the parts, upload a texture and then open: Texture - UV Map editor. Now you can move it and land it on a different colour.

GRAPHICS
Are also very intuitive, I'm not teaching how to draw them, I use nfshtool so I can easly see the transparencies (the images that end with and a). Take a look at the original court you are modifing ( the fsh file) and you'll see what goes where on the court. You also can see it and change the positioning with Oedit.
I learned this while patching live 2000 with the Tutorial from Thuan Vuu, seeing others work ( Bchogan has done a great work on live 03 courts) and practicing.