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very nice challenge but on my first try I got just crap ... Gimp starts with zero for the index what causes an off by one Error

This is one of the rare challenges which itself gives a hint how to solve it. It is surely a candidate for the challenge hall of fame.

I didn't understand the answer until reading this thread, due to the off-by-one problem - I got randomness out of highlighting prime values, but highlighting odd numbers made the bold bit quite legible, though still noisy. The actual solution, not in bold, wasn't quite readable. I then manually entered exceptions to the odd/even rule - all even numbers which I had to make it treat as odd, as follows in hex:
Based on JanB's post, I guess this is the set of numbers one less than a composite odd number.

These index values are from Python's Image module. The message at the top was written in zeros; the message at the bottom in 0xff. So it exercises both ends of the spectrum - I guess some image libraries use values from 1 to 256 then.

In any case, it sucks if different image libraries use different numbering schemes, but it's good to know about. I think there are other challenges on the site which only work with Python's scheme.

Funny challenge. I ran into the same off-by-one problem. After changing the prime palette entries didn't work out, I simply tried to change the palette entries of the pixels near the upper and lower letters. I was hoping to exclude the non-message pixel that way... and it worked. After 15 min the solution appeared automagically

nice challenge, but first i had the same problems like JanB...it was to late to solve

same problem like JanB with GIMP. Virus pointed out that the sentences mentioning 1 and 256 appear with index 0 and 255, so we gave a last try with idx-1, which worked

TweakPNG for the win! Easy as pie to manipulate the palette with it

Also, pretty good challenge this one.

This challenge was a great hassle for me:

I got the hint by using the index values from the IDAT chunk as colors (by deleting the PLTE chunk and changing the color mode to gray-scale). I wrote a small Java program to display the gray-scale image, it worked perfectly. Then i displayed only the 0 and 255 pixels, it also worked, i could clearly read the hint. Next i tried to display only prime (minus 1) pixels and got .. only nonsense.

So i assumed the solution must look differently. I tried a lot of things, but nothing worked. Finally i compared the data from the IDAT chunk to what Java had produced: the data was totally different. So far, I was not able to track down the reason.

As color 0 talks about 1 and color 255 about 256, I was sure what the encodding is ... so even I made 1 off error, I was sure it's my fault and second try was OK.

Loved this challenge! It took me few days until I understood the odd existence of the PLTE chunk. From then on, it was quite intuitive what to do. And of course, as everyone here, I made the off by one error with GIMP

I really liked this challenge. Did it together with Vralfy.
Extremely nice that 1 and 256 were highlighted ... or else we probably would have failed.
Did it by hand, with HxD ^^