The idea of the challenge is to get everyone out in their swimwear each month. Sometimes it's hard to drag oneself to the swimming pool or hot tub and a little challenge helps, giving the feeling you are part of something, although it always feels good once one is in the water.
The challenge also encourages those of us with swimwear collections to dig out colours and designs they haven't used for a while, usually sticking to a current favourite swim suit, and for those beginning thier speedo collections an excuse to buy new and experiment with how different suits look and feel.
As far as the challenge is concerned royal blue covers all mid blue shades between navy blue and light blue but avoiding the realms of turquoise and purple. As the challenge covers thongs, bikinis, briefs, sangas, square cuts and jammers your (Swimmin) simple blue jammers would probably count nicely, unless perhaps you'd like a pair of these .......
https://www.ebay.co.uk/itm/Mens-Swim...53.m1438.l2649
I'm rather partial to Addicted and the blue looks good.
(I recently bought pink ones and love them, though I haven't had the courage to wear them in public yet)
Great Britain Swimming teams traditionally wear royal blue.