Is there a finish/stain on the shingles now? If so, another stain will not work, since stain needs bare, unfinished wood on which to work.

Primers are used for paint, not colorants. Some woods stain better when treated with a preconditioner. That equalizes the differences in grain so the stain is absorbed more equally and the result is an even color rather than one with blotches. Examples of woods needing preconditioned are pine/spruce/maple/cedar/some oaks.