If you are struggling accepting that the speckled turtle you've seen is a pond slider rather than a western pond turtle, here are some examples of melanistic sliders:
https://www.inaturalist.org/observations/199303483
https://www.inaturalist.org/observations/197627830
https://www.inaturalist.org/observations/174342503
https://www.inaturalist.org/observations/26791671
I get it, these turtles have all the beautiful marbling and speckling that are the western pond turtle's trademark namesake (=marmorata). Don't be fooled; this is the melanism in action. It is bringing out secondary markings. These turtles are red-herrings without their namesake "red" ears and stripes.
You can learn more about what little is known about melanistic turtles in Jeff Lovich's 1990 conference paper on melanism in slider turtles, available at the link below; see page 238-239 (.pdf page 5 and 6) for images of shell and head markings, but before you click, be forewarned, page 6 is a little graphic for anyone squeamish: https://www.researchgate.net/publication/243655879_The_development_and_significance_of_melanism_in_the_slider_turtle [and select the blue (Download) option]
If I may summarize Lovich, ontogenetic melanization involves a "process of pattern reorganization" rather than any increase in pigmentation, and in Trachemys scripta first appears on plastral scutes, then the carapace, and lastly the skin of the legs, tail, head, and neck; in males in particular, this process proceeds as follows (see page 237; .pdf page 4):
SOURCE: Lovich, Jeff, William R. Garstka, and Clarence J. McCoy. 1990. The Development and Significance of Melanism in the Slider Turtle. Life History and Ecology of the Slider Turtle, pp. 233–254. In: J.W. Gibbons (ed.), Life History and Ecology of the Slider Turtle. Smithsonian Institution Press, Washington, DC, USA
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