You can code shaders in AppGameKit if you want to with syntax highlighting too
Go to Tools->Configuration Files->filetype_extensions.conf
add "*.ps;*.vs;" to the GLSL line so it reads
GLSL=*.glsl;*.frag;*.vert;*.ps;*.vs;
If its a very simple shader then editing it in AppGameKit is fine.
I also use
Shadertoy and
Typhoonlabs Shader Designer Tool as well as
glslsandbox.com
EDIT:
Also...cant believe I forgot this....
https://shaderfrog.com
A few people on here were asking about a shader editor for AppGameKit with a nice interactive plug together type front end (Ie..like unity) and shaderfrog has that and the shader produced can be used in agk with a small amount of extra work
