Here's the grid for Illusion Tulips (side 1). I came up with the pattern on my own, and sort of made up my own notation for this grid pattern, so you'll have to interpret (not that it should be that hard). In brief:

Each row in the chart represents TWO rows of a single color: knit across the first row. For the second row, knit where the color appears in the chart; purl where it is white. (Because the second row is the WS, the knits will show up as purl bumps, so the color will show up as part of the illusion.)

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I would be delighted to see anything anyone decides to make using this design (or inspired by it), but please do credit me if you use this. (This is the first time I've posted a pattern of any kind on the internet, so I'm a little nervous…
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Some notes:
I used black for the background color (appearing as gray in the chart for visibility), and light green, yellow, and red for the flowers, but of course any colors you choose will work. The illusion is strongest when there's a great deal of contrast between the foreground and background colors, so I wouldn't recommend, say, blue and purple together.

The illusion appears when you look at the finished square at an angle (above or below), because only the purl bumps show up, so the different colors form a picture. Looking at it straight on, you should only see stripes. So it works best when the finished piece is held pretty flat. That's why I put it on a pillow with a border of black, although even this wasn't perfectly flat. Of course, it could be just as interesting as a blanket or something, only you wouldn't see the whole image very often.

Also, your stitches need to be fairly tight, but not ridiculously so. I used Caron Simply Soft on a size 7 needle (although the size recommended on the yarn label is 8), which worked perfectly, especially since I have a habit of pulling my stitches very tight. I had previously experimented with size 6 needles, but these resulted in the purl bump barely sticking out at all. Basically, choose a gauge such that the purl bumps are very prominent, but the fabric is still a bit stiff. Definitely experiment with a very small portion of the pattern first.

Finally, I want to stress that it's not at all hard to come up with an illusion knitting pattern, even one involving more than two colors like this. It's quite easy with a painting program like Photoshop, but even by hand it could be done. Keep it simple, and start by drawing a grid with as many columns as you want, and half the number of rows (since each grid row will be two knitted rows). Lightly sketch your image, and fill in lightly with colors--foreground and background. (If you're going to use more than two, remember that each row is a single color all the way across unless you know how to work two colors at once, which I don't.) Then go along your grid, filling in darkly only the squares that you want to have show up in the final image. I can't stress enough, KEEP IT SIMPLE. If you find yourself filling in only isolated squares in a single row, they won't show up right, so thicken your shapes and cut back on details.

Er, so, this turned into an essay when I wasn't looking (why do my artist comments always do that?) so I'll just wrap up by saying that I think a world with more illusion knitting in it will be a better place, so go forth and be illusory!