Gavin seemed to take his eye off the ball and allowed a free-kick to slip from his grasp for the Rams' opening goal.
It was unfortunate for the 20-year-old after he had showed all the attributes expected from a goalkeeper in wet, windy conditions at the Baseball Ground.
In the end the home side ran out 2-0 winners to leapfrog us at the top of the FA Premier Reserve League.
Colin said: "To be fair, Gavin was one of the best players on the pitch up to that mistake and even after that, he recovered to help Anthony Gardner out of a tricky situation.
"Unfortunately, as a goalkeeper, if you make a mistake like that there are very few people covering up for you.
"When the ball comes into the box someone still has to be on the player that scored and I told the boys afterwards to make sure that the danger has gone before they turn away, don't always assume someone will deal with it."
Ironically Colin suggested that he thought we played better in the second half, when we actually conceded the goals, than in the first when impressive Derby carved out a few chances.
"We failed the test in the first half," he added. "Funny how football is, we played better in the second half when we conceded two goals.
"If we are honest, they should have scored in the first half, one, maybe two, because they had a few opportunities.
"From that point of view the first half was disappointing.
"We then ended up with the best chance of the first half, missed that and then another at the start of the second half, missed that and it ended up 2-0, which was a pity."
Colin Calderwood jumped to the defence of goalkeeper Gavin Kelly after his mistake against Derby County on Monday night.