According to Nintendo, these new Joy-Cons are expected to be launched alongside Arms which is due for a release on the 16th of June, so do keep an eye out if you like what you see. The Joy-Cons themselves are pretty straightforward, but it is the Joy-Con battery strap that is probably the most interesting.
Alongside the launch of #ARMS, the new Neon Yellow colored Joy-Con and Joy-Con strap will be available! pic.twitter.com/oFmMznYYWi
— Nintendo of America (@NintendoAmerica) April 12, 2017
From what we can tell this attaches a battery pack to the back of the Joy-Con that is powered by 2x AA batteries. As it stands the Joy-Cons last a pretty long time, so it is unclear if these battery packs could potentially help recharge the Nintendo Switch while attached to the console which would be a nice touch given that its battery life could be better, or if they are purely for the Joy-Cons.
There is no mention on how much the battery pack will cost, but the pricing on the new Neon Yellow Joy-Cons should be the same as the other controllers. Alternatively if you don’t mind spending a bit more, you could always check out ColorWare for more customized options.