Where do you live? Perhaps it is too early and cold for dill.

FataMorgana

I live in San Francisco Bay Area
same problem was here during last summer

I just planted seeds last october and they emerged as they liked... by now most of them are about 5-7 inches tall and getting discolored gradually...

Does your extension service do soil testing? If not, you can still contact them and get a list of labs that test the soil...you'll receive directions about how to take the samples and where to send them.

I suspect some soil composition problem.
Dills generaly like cool/spring-like weather and are frost hardy. Is your soil clay, that get hard? how about PH?
I think a soil analysis should help.