you only need to select Custom if you want to change the settings.
You can find here:
You can use coloring by expression to accentuate certain values. Supported formats: RGB, ARGB, and HSL.
Expression: Enter the expression that you want to use. Click "fx" to open the expression editor.
The expression is a color code: Selected by default. In most cases, it is best to keep this setting. When the selection is cleared, the expression evaluates to a number, which in turn is plotted against one of the chart gradients.
It doesn't allow me to create and/or use my own color palettes - this button show only some pre-defined color palette.
To be more precise - what I would like to do is named as "color scheme" (in Qlik), so I
would like to create my own color schemes and use it just by selecting.
I knew the option with coloring by expression - maybe it solves the problem, but this is really annoying to write a formula to use custom colors, especially when you have to create dozens of dashboards.