Can any one tell me why a certain big canal (the Albert kanaal in Belgium) have the colour white when other (smaller rivers/canals) are in bleu ? Can i change the colour ? How can i do that ?

What version of the map is that?

There are different maps on the device. But for the moment i only have the "GARMIN CYCLE MAP EU 2022.11 WEST enabeld.

From my point of view simply changing a colour won't help here.

The narrow blue lines (you see on your map) are simply waterway objects without a specification about the width of the "kanaal". See this Openstreetmap screenshot:

There should be overlayed multipolygon water objects with the suitable width. But those seems to be missing or for an unknown reason are not displayed in your case ... OSM screenshot:

And BTW, the exact same issue here on my Edge 1040 with MAP EU 2022.10 ;)

I spoke with Garmin and they said it would be addressed in the 2023.10 map when that comes out. I don't know when that will be.