This exercise is actually an adaptation of a research tool used in looking at the impact of conflicting tasks on attention and efficiency. When I was researching it, I kept finding references to something called "the Stroop effect" – I assume that's the name of the guy who figured out the idea. The page with the blocks of color should be mounted back-to-back with the page that has the words written on it. You want to put something heavy in-between, both so that it isn't too flimsy when you are holding it up for demonstration purposes and so that nothing "bleeds through" from the other side when folks are looking at it. In my experience, this is a great exercise for helping faculty (and others!) understand why students with (especially) learning disabilities and attention deficits may need extended time. Below is a "script" that I use when I am talking someone through the exercise. You don't have to feel obligated to stick to the script – it is simply offered to show you how I have used this in the past (and, by the way, I didn't think up this idea… I just stole it from someone else years ago!).
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Can I get a volunteer from the audience?

I have here a card with blocks of color on it. I want you to name the color I am pointing to. I am going to point to these blocks in order, but I am going to go at a fairly good speed, and I just want you to call out the color as I point to it, OK? Fine. (Run through all twenty blocks of color, moving in order from left to right, top to bottom; make it fast enough to keep them engaged and not so fast that they have any trouble keeping up. You WANT them to be successful on this run!)

Very good. Now… in a minute, I am going to turn the card over and ask you to do the same thing – tell me what color I am pointing to. This time, instead of blocks of color, there are words written here in different colors. I don't care what the word says, I just want to know what color it is written in. Just name the color I am pointing to, just as you did here. Ready? (Don't let them see the back side of the card until you say "ready?" and flip it over). Start in immediately and go at the same clip you did on the front side. You can badger them a little as you go – "no, that's red" or "come on, keep up" or whatever. Most people don't make it past the first line before they either give up or start to laugh and disrupt the task.)

That's a lot harder, isn't it? For the rest of you, what he is looking at is the names of colors, written in other colors. He is supposed to ignore what the word says and tell me what color the letters are. For those of us who are sight readers, that is very difficult to do, ESPECIALLY when I was putting pressure on him to do it quickly. Now, the task from front to back is EXACTLY the same – "tell me what color I am pointing to." I didn't subtract 25 points from his IQ when I turned over the card. I asked him to do exactly the same thing, but there was no way he could do it as well, as quickly, when there was extraneous stimuli given. For some kids with disabilities, that is exactly the problem they are facing when they are in a time-pressured situation. They have to sort out the extraneous stimuli and information in order to get at the task-at-hand. For some, they are receiving only partial information, or have difficulties in the input system that have them scrambling to fill in the missing pieces before they can respond. Either way, their faulty input/output systems don't detract from their basic smarts and their ability to accurately respond to the task – only their ability to do it quickly. THAT is why we provide extended time as an accommodation for some students with disabilities!