Thanks for your help so far.
Please excuse the fact that it was not quite clearly formulated. By the red filter, I mean something like red glasses.
With the red glasses, you can read the words I O I O I TEST I O I O I. You can't see anything with blue glasses.
My aim was that after changing the color, the words are still invisible to the naked eye, but then become visible with blue glasses. Red glasses would also not be able to reveal the words after my desired change.
Please excuse the fact that it was not quite clearly formulated. By the red filter, I mean something like red glasses.
With the red glasses, you can read the words I O I O I TEST I O I O I. You can't see anything with blue glasses.
My aim was that after changing the color, the words are still invisible to the naked eye, but then become visible with blue glasses. Red glasses would also not be able to reveal the words after my desired change.