Hubert Green, Dave Stockton and John Mahaffey share fourth at 3-under, while Dave Eichelberger, Jim Thorpe and Jesse Patino each shot scores of 2-under 70 at Coyote Creek Golf Club.
Snead needed a 10-foot putt to save par at the third hole but capitalized at the par-5 4th when he roped a 6-iron onto the green in two. He converted the three-foot eagle to go to two-under.
He quickly dropped a shot at the next when he played a 6-iron from the tee at the par-3 hole but three-putted from 25 feet for the bogey.
Snead ran off three birdies in a row, starting at the sixth. He rolled home a 15-footer at six and then a 12-footer at the next. Snead played a pitching wedge at No. 8 that landed 20 feet from the stick, where he drained the birdie to go out at 4-under 31.
The back nine was not as friendly to low scores as the front but Snead managed two birdies on the back. At 12, Snead got on to the fringe in two at the par-5 hole and then chipped to a foot for the tap-in birdie. He closed with a birdie at 18 when he hit a 9-iron shot to two feet.
'I played consistent today for the first time in a long time,' said Snead, who has not visited the winner's circle on the elder circuit since 1995. 'I haven't played well since I joined the Senior Tour. I'm not a good putter and I don't have a very good short game. It's a never-ending experiment out there.'
Nicklaus, who designed the Coyote Creek Golf Club, two-putted from 15 feet at the par-5 4th for his first birdie of the round. He added another at No. 4 when he wedged his approach to two feet.
The Golden Bear picked up his third birdie on the front after he hit a 9-iron to 15 feet. Nicklaus carded his first and only bogey of the day at the ninth when he three-putted from 35 feet.
Nicklaus pitched his third shot at the par-5 12th to five feet and converted the putt for birdie. He closed with a birdie at 18 when he drilled a 35-foot bomb.
'I don't have any complaints,' said Nicklaus, who shot a 68. 'My game wasn't perfect but I putted well and I drove the ball real well.'
Nicklaus is looking for his first win on the Senior Tour since the Tradition in 1996.
'I'm playing more now than I anticipated,' said Nicklaus. 'I'm in better condition, lost some weight, and my golf game started coming back. I love to compete. I don't want to hack it around and clutter up the field. That's not fun for me.'
Colbert could have tied Snead at the top of the leaderboard but bogeys at the final two holes dropped him into a tie with Nicklaus.
Green rebounded from bogeys at the first two holes with five birdies the rest of the way for a 69.
Larry Nelson and Hale Irwin are among the notable players at 1-under.
Click here for full-field scores from the Siebel Classic in Silicon Valley