Please consider design of  this medium - CAS/quat complex turns yellow with (siderophore) sequestration of Fe.  It's inconsistent to assume a darker blue color indicates a siderophore.  Darker blue color may indicate a pH change.

As Phil said, siderophore production would produce a yellow or orange halo - although a pale blue halo could indicate a small amount of siderophore production, the yellow may be covered up by excess blue. You should really use a positive control (either a known siderophore-producing organism or spotted purified siderophores) so that you can see what it should look like, and to be sure there isn't a problem with your media.

CAS agar plates don't work well with all organisms, it might be worth growing your isolates in liquid low-iron medium and then testing the spent media instead.

Thank you Phil and Alison for your suggestions

You shoud use positive and negative controls.

Also, the medium in pH-sensitive. You can check some info in the link below

http://jmbe.asm.org/index.php/jmbe/article/view/249/html_106

Dear All 

Thank you for your answer... Like Sirivastha I also got the similar results for siderophore production on CAS media. In my case some some bacterial strains produced orange yellow hallow, howevre I also observed slightly pale and blue hallow around some strains, and some colonies also look yellow orange of CAS media......... what you say which of my strains with pale blue hallow are siderophore producer in given picture..........please have a look on file attached

Please see link Xabier offered.  Blue - pale or otherwise is not consistent with siderophore production on CAS medium.

Different halo colors can be indicative of the type of siderophores produced. Blue to purple color changes can be linked to catechols, orange (hydroxamates) and yellow can be carboxylate producers.. But even at times, media can give a false positive so make sure you are removing the iron and using negative controls. 

Thank u Jones

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