Is there a difference between yellow and white speed signs in Massachusetts?

Yes.

If you look closely, the black and white signage reads "speed limit" while the yellow does not.

I'm nerdy, and I like to know these things, so I did a little research.

I remember as a kid I used to see "PED XING" signs and I was like how the heck do you pronounce that?! My mother kindly informed me that it was "Pedestrians Crossing".

In Massachusetts, and other states as well, there are "Speed Limits" and "Advisory Speed Signs".

The White: Regulatory Speed Limit Signs

Speed limit signage that includes black legend on a white background indicates the presence of a regulatory limit.

These are speed limits that have a special speed regulation and has been established in accordance with the Massachusetts General Laws and the MassDOT Procedures for Speed Zoning.

The Yellow: Advisory Speeds

Advisory speed signs have a black legend on a yellow background and are used to supplement other warning signs of a condition that may require a reduction in operating speed, such as an approach to a sharp curve or winding road.

Advisory speeds are not enforceable since their intent is to advise motorists of an appropriate speed, not regulate it. -mass.gov

Within the definition of the yellow sign, my next question was answered. Since it doesn't say "limit", it's not enforceable; however, I wouldn't be seen exceeding the speed by too much that what is displayed in yellow and black.