Martha Farwell (1901 - ) is an author for whom I have not yet been able to dig up any biographical information. She wrote two books, The Good Luck Bell, and this one horse book, Fiesta Colt.
Finding the book: reasonably easy to find in the USA. It was not printed in the UK.
Links and sources
Thank you to Lisa Catz for the photograph and summary.
Bibliography - horse books only
Rinehart & Company, New York, 1947, 176 pp, illus Robert Frankenberg
The story of two boys, a palomino horse, and Santa Barbara‘s famous Fiesta. First, Christy and his friend, Ramon solve the mystery of a palomino horse they find in a deserted barn. Christy helps Ramon get over his fear of horses, because Ramon’s aristocratic grandfather wants him to ride in the Fiesta parade. The two friends prove that no matter how different their backgrounds are, they have plenty in common when they share adventures and interests.