42051, is a sky blue synthetic triphenylmethane dye used as a food coloring.

It is not widely used, but in Europe it can be found in Scotch eggs,[citation needed] certain jelly sweets, blue Curaçao, certain gelatin desserts, among others.

An important advantage is the very deep color it produces even at low concentration, a disadvantage is that it fades fairly quickly when exposed to light.

In medicine, Patent Blue V is used in lymphangiography and sentinel node biopsy as a dye to color lymph vessels.

Patent Blue V is banned as a food dye in Australia and US, because health officials in these countries suspect that it may cause allergic reactions, with symptoms ranging from itching[5] and nettle rash to nausea, hypotension, and in rare cases anaphylactic shock; it is therefore not recommended in those countries for children.