Grace had two double bogeys on his back nine, wasting his four-stroke lead after two rounds. Colsaerts shot a 69. Grace and Colsarets head into the final round at 10-under 209.
After two days of benign weather, the Links course showed a glimpse of its fearsome reputation as the wind and intermittent rain made for a difficult day.
It took its toll on Spain's Pablo Martin. The three-time European Tour winner shot a 90 to sit last at 20 over.