Heya, lkpnjp
The problem sounds to me like your white balance is off. You can create custom white balance to offset the imbalance you seem to be getting. With the tree blocking the sun as you indicated, you had no direct light on the subjects. There are a bunch of white balance settings in your camera. You can try to use one of those or create your own by shooting a white piece of paper. What mode were you in? Some modes do not let you change balance (auto) I would say next time you get the orange/yellow tint, go to your white balance settings. Its ok to let the sun in the shot, just keep it behind you. It looks like you had the sun behind the soccer team and that means you were shooting into the sun?
Lastly, always shoot raw if your doing a shoot for clients. You can meter your shots in RAW mode. You can apply light changes as if you were there. Its much easier than filtering in Gimp or Photoshop.
Good luck!! Let us know if you resolved that issue. its fairly common in digital photography.
A picture is worth a thousand words.