dt 5.6.1 can open RW2 raw files, but in darkroom it says : Colors matrix “Panasonic DC-L10” not found and there is a black picture on screen.
Is there any thing to do (ART does the job nicely)
Yes, wait. 
Panasonic LUMIX L10 support by andriiryzhkov · Pull Request #976 · darktable-org/rawspeed · GitHub // Panasonic DC-L10 · Issue #21465 · darktable-org/darktable · GitHub
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You can always verify whether a given camera is supported here:
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Panasonic DC-L10 is not supported in 5.6.x yet.
There’s an open PR in RawSpeed
If you want support for L10 sooner and ready to hack a bit you can use those PR to modify cameras.xml yourself. That provides support for this camera.