Seen by hunters
Blaze orange is the bright orange that hunters wear so other hunters can see them. It is also called hunter orange or safety orange. In a test at Fort Devens, Massachusetts, from late 1959 into early 1960, about 1,100 soldiers were asked to name a range of test colors. Blaze orange was the only color that none of them mixed up with another color, even the men with color blindness.
In 1961 Massachusetts became the first state to require hunters to wear it, according to this history of hunter orange. Most American states now have some kind of orange rule for hunters.
Why deer don't notice it
Deer have only two kinds of color cells in their eyes. They lack the one that picks up red and orange light. To a deer, a blaze orange vest looks like a dull yellow or gray, so it stands out to people but hides fairly well in the woods.
About the code
There is no single official code for blaze orange. An American safety standard defines safety orange by a paint sample, and Wikipedia gives #FF7900 as a close screen match. The #FF6700 used here is a little redder, and it is the same code we use for safety orange. For a deeper, redder warning color, see international orange.