Slade thought the Mariners should have been given a spot-kick by referee Craig Hicks, after Frankie Kent’s challenge on goalscorer Charles Vernam in the area.
Grimsby had dominated the first half, with Vernam giving them the lead seven minutes before half-time when he clinically side-footed home from the edge of the area after Paul Dixon had latched onto substitute Scott Vernon’s flick-on and picked him out.
Karleigh Osborne rattled the bar for the visitors but Colchester improved in the second half and equalised less than two minutes after the break through Sean Murray, who lashed in from the edge of the area, after Drey Wright had found him following substitute Courtney Senior’s trickery.
Slade said: “I think we should have comfortably won that game.
“If it had been 3-0 to us at half-time, nobody in the Colchester camp could have complained.
“We ran the game for long periods in the first half and created four or five great opportunities.
“In the second period, they upped the intensity and got back into the game.
“But before they did that, we should have had a free-kick.
“The referee has missed that one and the biggest one of all is Charles Vernam in the last 10 minutes who has been bulled over in the box – it’s an absolute stonewall penalty.
“I’m fuming – it’s not a difficult decision for the linesman or the referee.
“We’re searching for that win and we’d have deserved it today – the referee has denied us a fantastic opportunity to take all three points.”
Colchester head coach John McGreal was pleased with his side’s improvement after the break.
McGreal said: “We were really poor in the first half and I’ve got no complaints whatsoever about us getting booed off.
“We didn’t perform well but credit to the boys, we made a little bit of a tactical change and I thought our top three got into the game a lot more.
“We started asking more questions of their defence, which we didn’t do in the first half.
“If we’re going to get beat, we want to get beat the right way and credit to the players, because they gave it a go and got something out of the game.
“Courtney Senior came on at half-time and started probing for us and Drey Wright got some width.
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