It could be, as Demi suggests, that "white" refers here to the white of official copy of the marriage certificate, as opposed to the blue "souvenir" copy retained by the couple. I do not know whether this practice exists in Mississippi, but even if it does, it seems to me that "white", in these terms, is a tautology: all official vital records are "white", so there seems no reason to specify that on the certificate.

I think this is to do with race, and with segregated record-keeping in Mississippi.

It comes as no great surprise to learn that, after the Civil War, "separate books for African-American marriages were kept [in Mississippi"
Alice Eichholz, Red book: American state, county & town sources, p. 367
http://books.google.es/books?id=chC81in93GUC&pg=PA367&lpg=PA...

There were "white", "colored" and "mixed" books:
"Miss Emma Giles married James Clearman on March 9, 1899. The record appears on pg. 219 of the Perry County [Mississippi] Colored Marriage Book covering the dates of August 17, 1897 through January 4, 1900, which is currently housed in the Forrest County courthouse system. The marriage books for the region were divided into either white, colored, or mixed races."
http://webpages.charter.net/djyontz/066.html

This was still going on up to 1960:
"Lauderdale County [Mississippi] MS Marriage Records Lookups by Elizabeth Hagwood
Marriage Records 1800 to 1960 (white book)"
http://www.rootsweb.ancestry.com/~mslauder/marrage/marr-04.h...

But in 1977, after the Civil Rights legislation? Apparently so, at least in rural Alabama, according to an article in the Lawrence Journal-World dated July 21st, 1991:
"A Civil War-era custom of recording marriages in books marked "white" and "colored" is still practiced in rural Chambers County.
At the red-brick courthouse, where the civil rights movie "Mississippi Burning" was filmed, probate office workers conceded that having segregated records seems outdated, but they said there is nothing sinister about it. [...]
In the case of interracial marriages, 'you have to write it down in both books', said one probate clerk."
http://news.google.com/newspapers?nid=2199&dat=19910721&id=S...

If the practice survived until the 1990s in Chambers County, Alabama, and only came to light by chance, it seems perfectly possible, indeed quite likely, that the same thing was still going on in Mississippi in 1977.