But what about the other way around? I mean if my opponent has all that protection how would I detect my opponent if I used IR sensor or Ultrasonic sensors ?
Same way as the other-other way around
If you make yourself invisible and the opponent does the same, either none of you can do a thing about it, or you can both see each other by other means, but then none of you are invisible after all... IOW, what you can do, so can he - pure logic so quit whining about it and spend your energy in making your 'bot as good as your abilities allow, mechanically, electronically and programmatically and be prepared to loose every now and then - it's very rare to see a beginners first 'bot turn out to be a win-all
I was reading about how IR sensors measure different distances for different colours?
Always post a link to what you need clarification on.
Could you please help me with the theory behind this? If it is black is the distance larger / smaller? and what about if it is white?
It's a matter of how much light is reflected and only valid for sensors that rely on the amount of reflected light to guess distance - they're crappy at best - if you want something that is fairly precise, no matter the color of the target, use a Position Sensing Device (PSD) - Sharp makes a number of PSDs with different detection ranges.
Also , What about ultrasonic sensors- If I decided to use that as one of my sensors and my opponent has covered his/her robot entirely with foam then how could I detect my opponent?
Again... Just like the other way around.
If your opponent use active detection, transmitting IR, US or whateverjust use that as a beacon (just like he's gonna do to you
Most Sumos (all?) will at least have IR for detecting the "edge", so if that isn't too hidden underneath it, just make a very sensitive IR receiver and aim for the light.