Like @evil8 suggests, this is really a matter of preference. I'm partial to standard red Neocaridina.Recently I've entertained the idea of adding shrimp to my planted tank to help make it a bit more colourful and lively, and maybe tank care of any detritus or mulm that accumulates in the tank that I cannot see. The tank has a pretty even mixture of plants with red, orange, and bright green in my opinion.
I'm hoping to get a shrimp in a colour that contrasts the best with my tank. So far I'm thinking red rili, black rili, orange pumpkin, bloody mary, fire red cherry, yellow, any colour besides green, blue or all black/white. I'd love to get blue, but some of my current tank inhabitants already have that colour, so I don't want to overdue it.
Right now I think the best colours would be red or black rili. I feel orange might get lost in my Rotala 'Orange Juice' as it takes up a good portion of the background, but I'm still considering it. Yellow isn't my favourite colour, but I think it could pop nicely among the greens, reds and oranges.
What do you think would be the best colour?
Could you post a photo or photos of your tank? That may help others make recommendations and help determine whether or not your tank is suitable for shrimp.
What kind of inhabitants do you currently have in the tank? How heavily is it planted? Do you have a lot of moss and cover? Any hiding spaces for shrimp? What are your water parameters? Temperature? kH? gH? Do you use CO2?
Keep in mind that any fish - any - can eat, maim, harass, harm, injure, kill shrimp of any size. If it can fit in their mouth? It can become a snack.
Update: Okay, I changed my mind a bit after thinking about it.
While standard red Neocaridina are still my pick... I think you should also consider wild-type Neos. Their coloration will develop a bunch of different ways over time and could look quite striking in a tank with plants of varying colors. You could even start with a few different color varieties and just let them do their thing. They'll begin to throw all sorts of colors and eventually morph into something more wild-type and darker. Could be one of the rare occasions where it makes sense to start with multiple colors - with the goal of them changing over time.