I can find paper titles like "(Main Title): A Case Study for California" but also "(Main Title): Case Study for California". Which form of the subtitle above (with or without the indefinite article) is more appropriate. Is there any difference?
Newspapers have their specific writing style for headlines, which is known as headlinese. Under this style, as CopperKettle has suggested, it is permissible to omit articles in many instances while writing titles and subtitles.
So, as an answer to your question, IMO, it is okay to write
"(Main Title): Case Study for California"
"(Main Title): A Case Study for California".
However, I don't say that this is grammatical, but, it is an acceptable writing style for newspapers' headlines. In India, such language of newspapers is known as 'block language'. The similar style can be adopted while writing headings and subheadings for research papers and scientific papers as Maulik V has suggested.
Check this question also to clear your doubts.