Refactor color dataset handling in ScatterplotPlugin #750

Job	Run time
prepare_matrix	5s
Cross platform build (Linux_gcc13, ubuntu-24.04, gcc, g++, gcc, 13, Release, Linux, libstdc++11, ...	5m 56s
Cross platform build (Windows-msvc2022, windows-2022, msvc-2022, 17, MD, Release)	8m 17s
Cross platform build (Macos_xcode16, macos-15, apple-clang, 16, Release, Macos, 16.2, libc++, armv8)	5m 33s
	19m 51s

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