Current color is #54595d. The default varies per-browser, but for example my Chromium uses #a9a9a9. The Vector skin uses #999999 for the placeholder in the search field (but doesn't override the default in browsers that support placeholders natively).
It's been made darker fairly recently in rGOJUb3380e8b70a3: MediaWiki theme: Normalize [placeholder] appearance x-browser and ensure a11y for consistency and accessibility. I'm not sure if the accessibility argument convinces me; the placeholders should never be necessary to instruct the user how to fill out the form, rather the labels should provide the necessary information and placeholders should be only for convenience.