Thanks to Margaret Rohdy for her help in obtaining these cards
The Handbook of the Library issued in 1927, advised that while "most of
the cards are white, it may assist readers to know that all cards with the
top edge coloured red and with the first line in red ink are subject
cards." Green, blue and yellow cards were used as subject cards for books
of Biography, Bibliography and Criticism respectively. Though the use of
coloured cards was soon abandoned, the typing of subject headings in red
continued. However, on computer-generated cards, subjects appear in
upper-case letters, while other headings are printed in a combination of
upper and lower case.
