In Sketches by Boz, Our Parish, Chapter III. The Four Sisters, Dr. Dawson, the doctor who attends Mrs. Robinson's delivery is said to "[display] a large lamp with a different colour in every pane of glass, at the corner of the row ...."
In The Uncommercial Traveller, Chapter XII. Dullborough Town, the UT says, "It is a mercy I have not a red and green lamp and a night-bell at my door, for in my very young days I was taken to so many lyings-in that I wonder I escaped becoming a professional martyr to them in after-life."
Apparently these colored lamps were hung outside the doctor's or midwife's (?) dwelling door at night to indicate that they were home, available to be "knocked up" to attend a birth during the night. Or, does Dickens mean that, because of all the lyings-in the UT attended as a child with his nurse, acting as midwife or nurse assistant to a doctor, the UT should have gone into the medical field to become a doctor, perhaps specializing in midwifery?
I can find no information in books or on Internet sites about 19th-century medicine, obstetrics, or midwives that can tell me about the multi-colored or the red-and-green lamp being signals for an in-dwelling and available midwife or doctor.