Is there 0-255 color standard?

So, I was tasked with job where I will write a C# application that will go through an image and list out the color of each pixel. Problem is, I'm not really sure what standard the color needs to be. The spec says:

"'px (a value in the result set)' shall be the pixel color at a specific spot of the image The pixel color shall be between 0-255, inclusive."

Later in the spec, it gives an "example of a 3x5 image," then proceeds to display 3 rows of 5 single numbers between 0 and 255.

Obviously it isn't asking for the Hex or CMYK values. It seems like it could be asking for the RGB, but it really seems like it's asking for a SINGLE integer value between 0-255 that fully identifies the color of a particular pixel.

Does such a standard exist?

This is an excerpt FROM the spec:

Here is an example of a 3x5 image file, “babydrawing.img”.

255, 6, 65, 78, 99

100, 25, 0, 45, 66

88, 190, 88, 76, 50