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Why are there just Azo dyes with colours of Yellow Orange and Red. Do the molecules become to be unstable with a greater pi-electron System? Every dye or indicator with other colours are'nt Azo dyes.

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    $\begingroup$ There are also blue azo dyes, e.g. trypan blue. You can have most any colour you want, all that is needed is the C-N=N-C linkage. $\endgroup$
    – porphyrin
    Mar 28 at 12:32

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Wikipedia article Category:Azo dyes list you all the azo dyes in the whole color spectrum, with colors explicitly indicated (yellow, orange, brown, red, blue, black) in the names of some of the dyes. So, the premise of your question that azo dyes ranges in only red to yellow-orange is false.

Also see: What is the origin of the colour of azo dyes?

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  • $\begingroup$ In addition to this, the presence of the diazo motif doesn't hinder the molecule to bind to metals; this opens additional variation including the colors perceived (example). $\endgroup$
    – Buttonwood
    Mar 28 at 15:27
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More complex azo dyes can be other colors, as with trypan blue. With its more complex structure comes more extended conjugation, which shifts visible light absorption from the shorter violet and blue wavelengths (which gives the more common yellow to red colors as the complementary color) towards absorbing yellow to red wavelengths (thus giving a complementary purple to blue color).

Structure of trypan blue: enter image description hereSource file

Typical case of staining an organic specimen with trypan blue: enter image description hereSource file

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