How is "cyan" pronounced and where is the stress?
Solution 1:
The Oxford Dictionary Online specifies the pronunciation of cyan as /ˈsīən/. With this pronunciation, the first syllable carries the stress.
Merriam Webster's online dictionary, on the other hand, specifies two valid pronunciations of cyan: /ˈsī-ˌan, -ən/. The dictionary entry in this case puts the stress on the first syllable for both pronunciations.
Having spent a good deal of time in both the UK and the USA, my experience with the word supports Merriam Webster's entry, but not fully. I've heard both pronunciations (/ˈsī-ˌan, -ən/) in colloquial and professional use, and I've seen the word cyan appear in poetry and spoken in certain dialects (especially Irish and Australian) such that the stress can be manipulated by syntactic structure. Your already-referenced dictionary discrepancies support this broad interpretation.
Ultimately, you're asking a very complicated linguistic question here, and there exists no narrow answer because a narrow answer necessarily depends on relegating the scope of the question to a specific dialectic region and even a specific usage example within that dialectic region.
Solution 2:
Here is my personal contribution:
I am Australian, and I pronounce it sai-ANN, with the stress on the second syllable. I asked a few friends, and they all agreed that this pronunciation was correct.
I asked a Belgian friend, and they gave SAI-an as the pronunciation, with the stress on the first syllable.