Maybe two new special cans "Copy" and "Paste"?
I like to use BobandRob's {I think} /pickColor mod.
It's really handy when you've got large colour sets, or want an allready used colour fast or if it's too dark to make out two different colours.
It's really good at allowing you to pick your variety of colours fast, the main problem though is how it effects the symbols and such on the screen.
Say you have default Red as your colour, the bricks in your brickmenu will be red.
You use the /pickColor command while looking at a green brick. Your Spraycan changes green.
Your brickmenu will still be red though.
If you then press E (Or your spraycan button) it will open up at the Red colour again though, which can defeat the purpose when you're trying to get a couple of close colours quickly, as you can only access one, then you have to scroll manually to get the other.
I guess in short, I could say I'm lazy, and still complain about the tools that help me continue my lazines. But I still like that mod for getting colours.
The game automatically changes the colours to match the new set.
Yeah, but if you do something stupid like choose a colourset that has absolutely no Red shades and then load up most builds, you'll have random odd colours in place of the reds.
Although, it's rather silly to go loading colourful saves if you have a colourset without many colours.