No reliable complete index exists of the Colotlan baptisms of the early 1800's -- not on Ancestry, not on FamilySearch, not in the church books themselves. I discovered this when I found records in the church books that I then tried to find using Ancestry and FamilySearch indexes, and many of them were not in any index.
While our family has a great many names in Colotlan, the immediate need was for Marquez baptisms in the early 1800's, which will be followed by the need for baptisms a generation earlier.
So I realized that the only way to find all the Colotlan Marquez baptisms was to go page by page and record by record through the hundreds of pages of church books and make a spreadsheet of all the baptisms of children of a Marquez parent.
That spreadsheet is now available at http://www.wwjohnston.net/famhist/#mexican -- be sure to read the "About this spreadsheet" worksheet. The work is not yet done, so that there will be more updates as time allows for the research.
Wesley johnston