Today I have been realized some drawings for my scientific article using PGF. After converting the DVI to the PDF file I have noticed that my grid which has been supposed to be gray was somewhat darker in Adobe Reader than Yap. Why do colors display differently in Adobe Reader (PDF version) compared with Yap (DVI version)?
In YAP you are probably using the dvips render method. This means you are actually viewing a postscript (PS) file (generated on the fly with dvips) rendered with the ghostscript of miktex and not a DVI.
Besides this: Different viewers (which actually show different file formats) can always be different. Don't forget that your screen uses pixels and if the width of a line is somewhere between two pixel numbers the viewers have to decide if to use one pixel more or less. I often had documents where lines appeared and disappeared depending on the zoom. And colors add an additional level of complexity.
YAP is a *pre*viewer. Don't expect its output to be exactly like the end product. Check sometimes the end product (at different zooms and if possible on different screens or - if you will print your document at the end - on paper).