Actually, mid-October might be a bit early for the Valley floor in Yosemite. But, the surrounding eastern Sierra will be prime about then. The park is much less crowded, accomodations easier to find, cheaper and the temps perfect. It is my favorite time of year to visit.
As you get nearer, check the following websites for fall color updates:CalPhoto
Click on the Fall Colors upate section...
Also, the Sacramento Bee newspaper does an extensive fall color update each year.The Sacramento Bee
This link has the archives for the last several years of this special section so you can see galleries of past year's seasons along with dates, etc.
Take care and maybe I'll see ya there!