Purple bottles turn this color in the sun "ultraviolet light" due to having "manganese" in the glass to make the glass clear. 1870 to 1914 is approx the correct age for purple glass. They stopped putting manganese in bottles in 1914 when they finally figured out why clear glass was turning purple. Your bottle is pre 1900. Great bottle! Yours is truly a sun colored bottle.
Be careful buying these purple bottles that are in antique shops or bottle shows. Especially if they are dark purple. People figured out if you buy any clear bottle from this date range, buy a less than $50 ultraviolet light at the box stores that they can speed up this color process. In nature with natural sunlight in may take many years to turn color. The cheaters can do it in a day or 2 in their garage. As long as they turn it purple they get a premium price because unsuspecting buyers assume that the bottle was laying in the desert sun for many years when actually they are being conned.
Be careful out there.
Gary from Oregon