Dark colors, like in our case Forrest Green, block about 99% of the UVs, VS White, which might block 50%. This is why you should always use a dark color on your sail covers. (or an opaque liner under white) The sails
will last much longer if under total UV protection... For bimini
/ awnings & such, The dark color will block more light & provide better shade, so your cabin
top & seats under this shade will be cooler. However the area just under the canvass (within 6"), will be hotter if a dark color, because it's higher heat absorption trumps it's Superior shade from UVs at this close range. We did experiments on a bright /
hot Fl Keys day, by holding a thermometer under a mini tarp of Forrest Green, and then Off White Sunbrella. At close range the Green was a bit hotter, but given a good breeze, not so. Once we moved the thermometer 8" away, the Off White was consistently hotter, due to the sun shining through.
If it is a mostly closed up bimini / enclosure, a dark color might be hotter due to no way for the absorbed heat to escape, however if it is a stand alone bimini or tarp with at least 6" of space overhead, the absorbed heat will be blown away in the breeze, and the Superior UV protection will make your cockpit
MUCH cooler. Especially when sitting down. (less squinting too).
a dark color makes a better awning.
On our hard dodger's Sunbrella covers, we learned this the hard way. After 5 years under Off White covers, the lexan
had serious UV degradation. After refurbishing, we made new covers. These were Forrest Green underneath for 99% UV blockage, & another layer of Off White sewn on the outside for it's Superior reflectivity. Best of both worlds! This worked great in this particular application, but it would be a beast to try to make a bimini this way. Plus it would probably be a moisture & mildew trap. It is too bad that they don't make a Sunbrella that is light on one side, but dark on the other, to make it opaque. There are Weblon fabrics & Sea Mark products that may come close, but they don't breath. Especially if you ever have enclosure curtains, you should have a bimini that breaths, like Sunbrella.
BTW... This same scenario applies to paint! If you want it as cool as possible down below, and plan to have your boat Awl Gripped, have it primed with enough GREY
545 primer that it is not then sanded micro thin anywhere, then go on with white top coat. White primer, which is standard, lets the sun shine right through, just like White top coat. Grey primer is 100% opaque which blocks the UVs, and the White topcoat gives the reflectivity. We painted a sample, about 2'X4', both ways... Half was White on White and the other White on Grey. Then with the sample facing the sun took measurements. The White on Grey was always a couple of degrees cooler.
Things that make ya go hmmm! Mark