if you mean the light blue base colour, the best bet as a starting point is to look the same colour but on a solid fabric (the jersey is quite open)
choosing the gilet, for the area to the right of the word athletica in this image photoshop gives the average rgb value is around bed4de
plugging the values into this tool http://www.perbang.dk/rgb/BED4DE/
gives a whole list of fits to various colour schemes including ral 'classic' (there are others, but ral on it's own implies classic)
there's no direct ral match for this, but if you took ral 5014 i think you'd be close then add a white like ral 9016 to the mix
this is not a good way to do it though, as it's relying on the original image colour being accurate and assuming no colour management confusion along the way
imho if you've got a real sample, it's going to be simpler to eyeball
why ral? are you getting paint made up?