The first 32 characters of ASCII are non-printable. There is no point in storing them in the atlas. To get the character code in cel7 you just have to subtract 32 from the ASCII code. For example, 'a' in ASCII is 97, in cel7 it would be 97 - 32 = 65.
The color in cel7 is 4-bit, which means there are a total of 16 colors. You can change the color of the foreground and background. If you need to change the color of the background, you have to add the color of the background multiplied by 16. For example, (color 5) will give a bright green color to the foreground and black to the background, while (color (+ 5 (* 3 16)) - green on the foreground and orange on the background.
Here is the palette:
