rish3 - 4:29 pm on Jun 26, 2013 (gmt 0)
Google filed a patent on detecting hidden links: [patft.uspto.gov...]
The part that might be relevant for you:
If two colors are not the same...[the algorithm] may determine that the two colors are similar if they are within a configurable range, or a predetermined range, etc. For example, when the colors are represented as color values, server 120 may determine that two colors are similar if their color values are within 50 (or some other value) levels of color of each other.
I assume "50" wasn't completely arbitrary, though it does say "or some other value". I'm also assuming they mean the sum of the difference across the R, G, and B spaces. Some other context I didn't quote implies that the 50 is a base 10, not hex.
You mentioned grey on white, so if it's truly a white background (#ffffff), the minimum grey would be #eeeeee. Personally, that still looks too subtle to me. The color #dddddd on white is clearly readable to me. That represents a decimal difference of 34 for each of R,G, and B, for a total difference of 102. That's twice the example cited in the patent, so I'm assuming it's safe.