Doesn't look all that fungal and wiki says there's a thing called yellow nail syndrome that's my two cents!
I strongly second the possibility of this being 'yellow nail syndrome'. With that said, I am keeping a few more differential. First up, Diabetes mellitus most often evident on the distal aspect of the nail of the hallux The skin and nails in some diabetic patients may have yellowish hue. Clinically one appreciates yellow nails of diabetes best on examination of the toe-nails. Possible mechanism - yellowing of the nails and skin probably represents glycosylation end products. a) Nails - Whereas keratin of the epidermis is only present for one month before being shed, that of the nail plate may be present for greater than a year. The protein- glucose reaction presumably continues to evolve in the aging nail resulting in the most yellow pigment at the distal aspect of the slowest growing nail. It is not a specific finding in diabetes mellitus since it can be occasionally observed with normal aging. Like the yellow hue appreciated generally in the skin of persons with diabetes, the significance of this observation is undetermined. The obvious question is whether or not yellow nails and yellow skin can be used as an quantifiable indicator of the degree of nonenzymatic glycosylation for other tissues of the body. b) Skin - It is known that proteins which have a long turnover time, such as dermal collagen, undergo glycosylation and become yellow. One of the advanced glycosylation products which has been identified, 2-(2-furoyl)-4(5)-(2-furanyl)-1H-imidazole, has a distinctly yellow hue. Yellow skin is a common finding among patients with diabetes, probably best appreciated on the palms and soles because of sparse competition with melanocytic pigment in these areas. There is currently no significance associated with this finding other than that of a time proven observation. ref - diabetes mellitus: review Other differentials Yellow discoloration of the nail could be a normal finding in elderly or develop secondary to jaundice, carotenemia, ochronosis or systemic drugs like hydroxyurea (yellow-brown discoloration), tetracycline, penicillamine, antimalarials, gold. Few dermatological conditions which may give rise to yellow discoloration of nails include onychomycosis, psoriasis, and alopecia areata. ref - Nail dyschromias Mendiratta V, Jain A - Indian J Dermatol Venereol Leprol
