That tarp would be better off being white side "out" to reflect suns rays away better and reduce heat? The same actually applies to car colour - Black cars actually get hotter inside than white cars when sitting in the sun as well (significantly hotter in tests)
My tests with brown side out vs silver side out did not show much diff.
As somebody else observed, some air space between tarp an hull might be an important factor.
But I found the most important factor was the type of tarp. Home Depot 10-12 oz canvas being vastly superior to the brown/silver plastic stuff - in that it lowered the temp instead or raising it.