The Ada mode is made to help you understand the structure of your source files. Some people like having colors or different fonts depending on the context: commands should be displayed differently than keywords, which should also be different from strings, ...
Emacs is able to display in a different way the following syntactic entities:
This is not the default behavior for Emacs. You have to explicitly activate it. This requires that you add a new line in your .emacs file (if this file does not exist, just create it).
(global-font-lock-mode t)
But the default colors might not be the ones you like. Fortunately, there is a very easy way to change them. Just select the menu Help->Customize->Specific Face... and press Return. This will display a buffer will all the "faces" (the colors) that Emacs knows about. You can change any of them.