in the Node Editor what is the data format coming out of the Color output of an Image Texture Node? Does the Color Space setting of the Image Texture Node change this at all?
The yellow color sockets output three scalar values, one for each of the color channels Red, Green and Blue. The output information is in a linear scale.
The Color Space on the image texture node will determine how the color values are interpreted. Images in display referred formats (like png, jpg. tiff and most video formats) are presumed to be in sRGB color space. Such images are encoded using a transfer function (sometimes called "gamma curve") that has to be linearized to to work properly in blender. Blender color and light operation need to be done using a linear scale, so using sRGB as Color Space is the correct choice. The linearization works by applying an inverse sRGB color transform to counteract the baked in curves.
For images used as maps, such as bump maps or normal maps, that are presumed to use linear values, with no transfer function (or "gamma curve") baked in, the correct setting is Non Color Color Space. When using this setting there will be no color transform applied to those elements on the render path.