Reddish-brown wax or black spots on the tail

Ferrets sometimes get tiny black spots on their tails, often
accompanied by a reddish-brown waxy deposit and hair loss.  They look
a lot like blackheads, and in fact that's probably pretty much what
they are.  Gentle cleaning, perhaps with a medicated cleanser (a
dilute benzoyl peroxide shampoo or cream will work better than ones
with coal tar or sulfur) that your vet can recommend, should help,
though it may take many weeks.  Often this is a seasonal problem that
clears up on its own in a few months.

Orange-speckled, crusty patches

Dr. Bruce Williams, DVM, says:

    An orange, flaky discoloration of the skin is a very non-specific
    finding in the ferret.  The crustiness of the skin means that the
    skin is not coming off in small microscopic flakes (1 to several
    cells at a time) like normally happens.  When you see a crust - it
    means that the normal way that a ferret sheds devitalized
    epidermis [dead skin] has been impaired.

    As far as the cause - there is not just one cause.  Many things
    can cause this change - skin parasites, fleas, ear mites,
    bacterial infections of hair follicles, fungus, endocrine disease,
    even distemper.

    Minor skin disorders such as these are more common with age.  They
    may be exacerbated by poor husbandry, or excessive bathing (more
    than once per week to ten days.)

    Most cases are due to a very superficial bacterial infection which
    will respond well to a weekly application of a gentle bactericidal
    shampoo.  Other tests that can be done at the time of diagnosis by
    your vet would include a skin scraping and fungal culture.  Should
    all tests turn up negative, and a four-week course of topical
    therapy not help, then the next step would be biopsy and
    submission to a pathology lab for microscopic examination.

Allergies are another possibility; and the area around bites, whether
caused by fleas or another animal, may take on a pink or orangish
color from dried blood.