I certainly can't answer all of these questions, but here are some speculations. According to the literature dermatophytes can produce melanin. Youngchim, S., Pornsuwan, S., Nosanchuk, J. D., Dankai, W., & Vanittanakom, N. (2011). Melanogenesis in dermatophyte species in vitro and during infection. Microbiology, 157(8), 2348-2356. Perhaps part of the differences in pigmentation you are observing is from different stages of pigment development, or different redox/stress conditions affecting melanin or other pigment production. One class of melanins called pheomelanins can be red in color: Eisenman, H. C., & Casadevall, A. (2012). Synthesis and assembly of fungal melanin. Applied microbiology and biotechnology, 93(3), 931-940. The melanins would likely be non-diffusable, but I have also seen reference to diffusible colored pigments in dermatophytes. This could also explain rapid change in color. KOH could react with and even clear certain pigments, changing their color. A literature search could identify the classes of pigments and their likely reactivity with KOH. Good luck. 

yes melanin is their

My Dear , dermatophytes fungi can produce melanin such as pheomelanins and it could not diffuse .

Best regards

Dear Vasishta,

In addition to my agreement with the answers of colleagues above, please check the following resources.

Best regards

SM Najim

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3167886/pdf/2348.pdf

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4687393/pdf/fmicb-06-01463.pdf

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4318813/pdf/nihms653871.pdf

http://www.sciencedirect.com/science/article/pii/S0022202X15497374/part/first-page-pdf

diffusable melanin gives it color

Dear colleague, 

- In dermatophytes such as Trichophyton rubrum and T. interdigitale, pigment usually diffuses from the hyphae into the agar. It is secreted by the mycelium, However, some species seem to have a mycelium which is truly coloured by pigments (http://www.mold.ph/trichophyton-violaceum.htm), at least in vitro.

- The different pigmentation on the obverse and reverse probably is due to the fact that secreted pigments require a given substrate to be produced (nutrients in the agar medium). The nutrients are taken from the agar and coloured metabolites are released into the medium. The obverse usually appears whitish in species such as T. rubrum and T. interdigitale because "from above" one only sees aerial hyphae which are not in contact with the nutrients from the medium. Aerial hyphae in these fungi are mainly "dedicated to" produce conidia.

- I do not know the chemical nature of the pigments in dermatophytes, but in general terms, the ability of a given fungus to produce pigments is greatly affected by the substrate where it grows. For example, many derrmatophyte strains which do not produce pigments on Sabouraud agar, can be stimulated to produce them on Lactrimel agar or on media with sterilized rice grains. When growing in human skin, the fungus has completely different conditions than in the lab. Moreover, I guess that if any pigment were produced in vivo, its colour might be masked by the natural changes in skin colour induced by the immunological response.

Success in your research!

Thanks for the answer!

The pigmentation on the surface is usually due to pigmentation of the spores while that on the reverse is caused by soluble pigments in the medium and the pigmentation of the vegetative hyphae.