Poway Progress, July 21, 1894, p. 4.
It was a good week in the county for rain, and also for rainbows. With the sun intermittently coming out between the showers in my neighborhood, I observed four or five in one day. This, naturally, led me to think of the small valley near Fallbrook called Rainbow (yes, readers, this is how my brain works). And I remembered my surprise at discovering that this valley didn’t get its name from anyone ever seeing a rainbow over it or anything like that, but was rather named for a real live person: J. P. M. Rainbow.
James Peebles Marshall Rainbow, usually referred to as J. P. M. Rainbow, came to San Diego County from his native Pennsylvania in 1875. He settled in the Fallbrook area with his wife Augusta and began growing fruit.
He apparently did pretty well as a farmer.
“Mr. Rainbow had the honor of shipping the first carload of fruit that was ever shipped from this county,” stated an article in the Poway Progress newspaper in July, 1894. The shipment “consisted of muscat grapes grown upon his twenty-acre ranch near Fallbrook.”
While working at his own ranch, the article noted that Rainbow “has found time to assist in locating a colony of twenty other families in its neighborhood, and today their holdings are largely planted in deciduous fruits.”
Rainbow also found time to serve in county government. He was elected to the county board of supervisors for two terms, from 1882 to 1884 and again from 1891 to 1895.
When J. P. M. Rainbow died in 1907 at the age of 71, the San Diego Union saluted him as “for many years a well-known and respected resident,” and concluded its obituary by stating “The town and post office of Rainbow, near Fallbrook, was named in his honor.”
The Rainbow post office closed in 1914, but the name remains.
Sources for this post included historic San Diego and Poway newspapers, the book San Diego County Place Names by Leland Fetzer, and the database Records of Appointments of Postmasters, 1832-1971 at the National Archives.
Get Updates Automatically-Become A Follower of the San Diego History Seeker
You can get weekly updates of San Diego History Seeker automatically in your email by clicking on the “Follow” button in the lower right corner of the blog page. You’ll then get an email asking you to confirm. Once you confirm you’ll be an active follower
San Diego History Seeker 
“An industrious and zealous exponent of the advantages held out to each actual settler by the country contiguous to San Diego.” Is he talking about Mexico?
I think the writer was referring to the county beyond the city of San Diego. Rainbow, after all, farmed in the northern part of the county and was a big booster for the county in general.