In Cairns (17 degrees South), Uca vomeris can be seen on the small sand flats at the mouth of Barr Creek or Thomatis Creek. Larger males have an almost white finger on their nipper and a partially or mostly blue back.
Uca vomeris often shares its habitat with other species that can look quite similar, especially in the juvenile stage. The photo above is Uca elegans, which was engaged in threat displays with the individual below, which is a Uca vomeris.
Gove in the Northern Territory is at 12 degrees South and I captured only one individual Uca vomeris as this species is quite scare and also the ground at Buffalo Creek is very hard. When taking the photo, I was stuck by how intense the colours were in comparison to Cairns crabs.
These individuals from Cairns were captured on the same day and within metres of each other. Some of the variation is size related with white being more common on larger individuals and beige mottling being more frequent on intermediate individuals.
I have just found out that this issue has been researched by experimental biologists. The title of the paper is 'The variable colours of the fiddler crab Uca vomeris and their relation to background and predation' and it was published in 2006 in 'The Journal of Experimental Biology 209, 4140-4153. Fortunately you can download the text for free by clicking on the title above.