We've noticed that the majority of power Blockly developers add coloured elements to the toolbox categories. Open Roberta, OzoBlockly, and App Inventor are all examples of this.
As of today one can now specify category colours (as hue) in the toolbox XML:
<category name="Logic" colour="210">
<block type="controls_if"></block>
<block type="logic_compare"></block>
</category>
<category name="Loops" colour="120">
<block type="controls_repeat_ext">
</category>
Here is the new toolbox:
Like the simplicity of a clean menu? No problem, the old toolbox is unchanged if there are no colours defined:
Feedback very welcome. Documentation will be updated soon.
--
Neil Fraser, Software Engineer
Google, Mountain View, California