Officeboy, Peco's solution hides those cells. (They're not gone, they just have their display width reduced to 0; you can get any of them back by reversing that.) That may be the way you like best to accomplish it.

But you did say "grey out". If you meant it literally, the only way I've found to do it (and it's a technique I used to use often) is to grey out the entire worksheet, then ungrey the columns at the left that I'm actually using. This allows me to see the helping columns at the right while providing a strong visual cue to my eyes which columns are which.

To grey out a whole sheet, select the entire range (hit <Shift-Space>,<Ctrl-Space> or click the button in the upper left corner), then use the Format Cells dialog with "Fill" colors. Pick the shade of grey you want. Then, as I said, change the "usual" columns back to "No Color".

When you select a fill color, that cell no longer has the mild grey border that marks it from its fellows. Most people can live with that, but I usually change the borders on such cells back to a light grey, which is the one that matches uncolored cells. You can make your own mind up about that.