Thats an interesting little illusion, fortunately I saw the 74. I bet without my contacts though I wouldn't have been able to see either number.
I can identify the object but when it comes in color definitely not. I agree with your introduction at first I saw 21 inside but as I read the intro then I look it again then I saw the 74.
This is clearly 74, everyone seing something different or complaining that it's not well done have a visual problem. I can understand that you want it to be 71 since half of the 4 is with other colors, hence to make color-blind people fail at this test.