@mpaland With custom colors the obvious choice is [style.background-color] as you already mentioned.
AFAIK the button’s color attribute in Ionic 2 is intended to be used only with names of colors defined in the $colors list in app.variables.scss. The fact that it’s not working with color values is not a bug, it’s a design decision. And this makes sense because its goal is not to save a few keystrokes (compared to using [style.background-color]) but to allow the use of predefined colors (i.e. the ones from the $colors list) which are otherwise not easily accessible in the code.
I guess that the docs should be updated to clarify this.