Cole slaw can be purchased as either a dry mix of chopped cabbage (and perhaps other veggies), or dressed and ready-to-eat. In either form, the cabbage appears white. Yet a full head of cabbage appears green. Why is there a color difference?
There is a transition in color between the outer cabbage leaves and the inner ones. A few on the outside are green, but they become significantly paler as you move towards the inside of the cabbage (see cross-section at right below). Also, the outermost leaves may be removed and discarded before the cabbage is processed (they're most likely to have dirt, blemishes, insects, etc. from growing outside).
Since most of the cabbage's volume is in the inner white leaves, the majority of chopped or sliced cabbage will look white. (Adding white dressing in a coleslaw will lighten things up even more, masking pale green leaf shreds.)
There are also varieties of cabbage that are whiter than others (e.g. white cabbage), which will appear even paler. But even red cabbage has some white on the inside!