Possible Clues:

• It's red to start 3 down, not blue to have lighted on the money up there
• Red for a C.I.D.?
• Test paper?
• Word before paper or test
• Having settled problem, turned over paper for chemistry test
• Fifty-one must be under it for red and blue
• Animated by flipping problem dye encountered in testing
• For a CID take it as read, by the sound of it
• __ test: pH indicator
• Take it as read, by the sound of it, for a CID