Hi @process - as far as I know this is not possible. I primarily think this because Omnigraffle utilizes OSX’s color picker, which does support the ability to add colors. If you click on the color rectangle instead of the arrow, or click cmd+shift+c you will prompt the color picker to appear. You can select your desired color, and then drag it from the large rectangle at the top to one of the empty white tiles at the bottom to save the color.
Why offer a list of colors to the user without the ability to modify that list to a palette that is relevant to the diagram the user is creating. If there legitimately is no way to change this then it feels more like a vestigal feature, something that was only partially developed, than an intentional design. If they really intended the OSX color picker to be the sole tool then why provide a list at all.
I’ve been hoping to be able to customize the drop-down color list since version 6 came out. OG is now at version 6.6 and this default color list still doesn’t seem to editable. It would be great to be able to customize the list or base it off a palette from the color picker so that frequently used colors were most accessible from this part of the interface.
That Other is the Mac OSX Colour Picker. It is fully customisable. In one place. Such that your custom colours are accessible from any app (one of which is OG). That means, once you customise it, you will never have to repeat the process. Instead of customising colour pickers within each app, which would have to be done once per app, and redone when the app is upgraded, etc.
Further, it contains a variety of colour pickers: colur wheel; colour sliders; colur pallettes; etc. There is virtually no limit to the (a) colours you can add, and (b) the way in which you approach colours.
Once you use the OSX Colour Picker, from OG, the window remains in the foreground, like the Inspector. Therefore, after that first click, to get the Colour Picker Window, you never have to click again to obtain it (unless you overlay the Inspectors, etc). Therefore, it is the same number of clicks and movements to get to the Colour Picker from OG (not counting that first click) as to use the in-app colour picker.