Call me dumb, but I'm struggling to see what the problem is.
First, as we all know, most clubs struggle to fill three Colts teams. So they're not in the business of turning away keen young blokes who turn up to train. All grade clubs have websites with contact numbers. Ring them up!
Secondly: the thing is, you've left school. If you have been playing in GPS 1st XV Rugby, it's been easy. Someone comes along and puts you in a team. Training is all laid on for you at school, and you get cheered when you run onto the field and everyone tells you how great you are. But once you're out of school, that all stops. You need to get off your backside and motivate yourself to expose yourself to an unfamiliar environment, with new people who aren't specially impressed by your GPS 3rd XV rep jumper, and get yourself to training on cold, wet nights, and drive yourself to games at Forshaw or Penrith. You actually need to care enough about the game and your performance to do the work yourself. If you're inclined to complain that no-one has made your transition to Colts easier, then maybe you'd be better off playing for your school old boys team, where you can tell stories about those great games against Scots or whoever.
Thirdly: it's a new game. I was a distinctly average player, but I played in Colts in teams higher than a couple of guys who had played Australian Schools, because they were cruising on their school reputations (this is less likely to happen these days, I admit, because the guys who represent now have their eyes on professional contracts and tend to take things rather seriously. But the point remains - what you did at school counts for very little once you leave school).
Last point: people who like to complain that they don't get a fair go in Colts really just need to stick at it. Even the low-turnover clubs like Easts and Gordon used nearly 40 players in Firsts Colts last season. In all my years of involvement with club sport, I have never seen a player who performed consistently well and did not get rewarded for it. I was playing at University in the 1980s when Rob Egerton showed up. No-one really knew who he was - just some skinny guy from Canberra who'd played ACT Under-19 cricket. There was a spot available on the wing in Colts Threes. And that was his first game. He didn't whinge, or carry on, or tell everyone that it was unfair, because he could actually play better than just about everyone else in the Colts squad. He just went out there and ran around people. He let his form do the talking. Rather later, Tim Davidson started in Thirds Colts. I'm pretty sure Luke Burgess got his first run in Fourths Colts. Where you start isn't important. How you respond to where you start is what matters.