Can i amend some of the colors used on sleep graph? Blue and dark blue are difficult to be distonguished... i would like dark blur in yelloow to bold out :)

No, you can't, the colors are not customizable. However, you can easily distinguish the individual sections even without colors - by the height of the columns.

Same issue. I cannot distinguish between the blues, and I don't understand why colors this close had to be selected. 

This seems like a problem that would be easily solved by picking better colours. It's also not helpful when I look at it across a broader context of multiple weeks.

1175949 said:

This seems like a problem that would be easily solved by picking better colours.

That's true, but we cannot help you with that here on the user forum. I recommend you to suggest it directly to Garmin at Share Ideas | Garmin.