In its most widespread sense, "purple" is not a part of the spectrum.

Normally "purple" is used for colors that are equivalent with a mixture of red and blue, so that none of the spectral colors can match that mixture.

"Violet" is also used for colors equivalent with a mixture of red and blue, but which have so much more blue than red that there are monochromatic colors in the spectrum which can match that color and which have a frequency higher than the monochromatic colors that appear blue.

The fact that the monochromatic colors in the violet range have the same color as a mixture with a lot of blue and a little red is caused by the fact that the human red photoreceptors have a defect, instead of filtering only red light they have a second peak of absorption in the frequency range of the monochromatic violet light, so the violet light is perceived by both the blue photoreceptors and, with an attenuated value, by the red photoreceptors.

The acronym ROYGBIV should better be forgotten, because it is not known with certainty what Newton meant by "blue" and "indigo", so this acronym is misleading for the people familiar only with modern English.

It is pretty much certain that Newton did not use those words with their present meaning. He used them based on the labels of some artistic paints that he happened to have at home. His indigo paint (i.e. with indigo pigment) appears to correspond with what today would be called "blue", while his "blue" paint seems to have actually been a blue-green paint (i.e. the color that is complementary to red, and which is also called correctly turquoise and frequently but incorrectly cyan). Thus it appears that Newton meant to say that the 7 spectral colors are red, orange, yellow, green, blue-green, blue and violet, which makes sense.