Whether a flash is permissible is only part of the challenge. A built-in flash would be inadequate anyway... you'd need a strong external flash.
You've dialed in some unintended exposure compensation. You'll want to set the exposure compensation back to neutral.
When you're in an automatic mode, the front dial usually controls the mode-specific feature (e.g. in Tv mode the front dial controls the shutter speed) and the rear dial controls "exposure compensation".
"exposure compensation" means that when the camera meters to determine what it thinks would be a correct exposure, you'd like the camera to deliberately over-expose or under-expose the shot. You would do this because you're not satisified with the exposure the meter suggests. The meter will usually be wrong if the scene is dominated by "black" tones or "white" tones rather than things that work out close to "middle" tones. A camera will tend to under-expose when shooting white subjects in the snow. It will tend to over-expose when shooting dark subjects in near blackness. This is because the camera "thinks" all scenes shoudl average to a middle gray tone (most of the time that's actually true). But if you get a scene where that general assumption isn't true, the camera will get the exposure wrong. SO... the camera allows you to correct the computer by telling it how much you think it should adjust the exposure from whatever the meter would normally have done.
If you're standing on the side of the court then you are probably about 25-30' away from your subjects.
Switch on your camera, half-press the shutter button to get it to meter a scene, then use the exposure level indicator -- the scale that shows the -3..-2..-1..v..1..2..3 with an arrow indicating the exposure level (there are three of them.... one is optionally displayed on the rear LCD, one is always displayed on the top-LCD panel, and one can be seen looking through the viewfinder.) Roll the REAR DIAL to center the arrow back to the middle of that scale. This will ONLY be possible when you wake-up the metering system by half-pressing the shutter button (or any other button you've configured to make the camera start metering.)
When shooting action, you usually want a minimum of 1/500th sec shutter speed. In some situations, 1/250th may even be fast enough (but usually 1/500th). And sometimes it needs to be 1/1000th or faster.
I can see by looking at the EXIF data in your 2nd shot, that you actually DO have adequate lighting to do ISO 3200, f/2.8 and 1/500th. The shot you posted was taken at ISO 3200, f/5.6 (but with a lens that can open up 2 more stops to get to f/2.8) a shutter speed of 1/100th (but if you open the aperture by 2 stops you can speed the shutter up by 2 stops and that gets you to 1/400th)... but the shot was over-exposed by +2/3rds of a stop (meaning if remove the exposure compensation you actually could run the shutter speed up to 1/640th and STILL have a good exposure.)
At a 50mm focal length (your first shot was at 42mm), f/2.8 has a depth of field of just over 8' for a subject focused 25' away. Depth of field at f/2.8 could be an issue if your subjects are spread out across a larger range of distance. When you go wider (e.g. your 2nd shot was at 26mm) the depth of field increases dramatically (about 56'). You'd just want to be aware of that when shooting at f/2.8. You can use a website such as DOFmaster.com to look up these things (you can also get smart-phone apps that will calculate it for you when you're out shooting.)
Tim Campbell
5D III, 5D IV, 60Da
