Red-orange is a compound term, and compound terms are how every language handles the parts of the spectrum for which it has no single word.
The pigments that sit here were among the most used in the medieval workshop. Minium, red lead, is a warm orange-red produced by roasting white lead, cheap enough for ordinary work and the standard colour for the initial letters and rubrics of manuscripts. Realgar, arsenic disulphide, gives a hotter and more brilliant version at considerably greater risk to the person grinding it. Both were in constant use, and neither was described by a word meaning orange, because no such word was yet available.
That is the substance of the entry. The study of basic colour terms, established as a field by Brent Berlin and Paul Kay in 1969, found that languages acquire such terms in a strongly constrained order, beginning with dark and light, then red, then green and yellow, then blue, and adding brown, purple, pink, orange and grey later and inconsistently. A language at an early stage does not fail to see the hues it has no words for; it groups them differently.
Red-orange in a modern colour system is a coordinate, a defined position on a wheel between two named anchors. That is a nineteenth and twentieth century way of thinking about colour, coming out of the colour order systems built for printers, dyers and paint manufacturers. It belongs to a different intellectual project from the symbolic schemes recorded elsewhere in this atlas, and the two should not be read together.