Yes, bright ambient light reduces contrast, so going with a black and white display will be the best you can do. Increasing font size and weight may also help. Hue differences also tend to wash out in bright light so your color codes are probably performing poorly anyway. Tinted sunglasses probably don’t help either. If the color codes are very important, then you should crank up the color contrast by maxing out the saturation for outdoor use, rather than use the relatively muted colors found in Office 2007.
However, perhaps a better solution is to supplement your color codes with other graphic codes like weight, shape (e.g., icons), and density/number (e.g., dashed versus solid lines). For example, a border of high-contrast diagonal line segments suggests urgency for many users. Distinguishing multiple levels of shading is also more difficult in bright light, so I wouldn’t rely on that. Using redundant graphics in addition to color will maximize you apps accessibility as a side benefit (and your case just goes to show we’re all disabled under the right circumstances).
Any chance your user is using polarized sunglasses? Since LCD screens use polarizers, such sunglasses can make your screen very difficult to read in any kind of light. You could advise the user to try different sunglasses.
To make it easier to read across rows of the table when the sun washes out your zebra striping, you could insert a half-high empty row every 4 or 5 rows. Or you can try a horizontal rule every 3 to 5 rows.