The Color Mixer is available by tapping the 3rd button from the left in your toolbar. It has a familiar design consisting out of three main parts. The top rectangle as shown in the left image is showing shades of green has a small round cursor currently at the very top right corner. You can drag in it to adjust the color. The middle section looking a bit like a rainbow is called the gradient strip and has a cursor consisting of two opposed triangles. You use it to select the approximate color neighborhood you want to set. The current draw color is indicated in the row of buttons at the bottom. Here you have five buttons corresponding to your document's color palette. In the left image above we see the default color palette and in the right image we see a custom palette with only gray shades (as used in the Grays document).
To set one of the five colors in your document, you open up the Color Mixer and tap on the bottom row button corresponding to the color you want to change. Then you adjust the color in the top rectangle If you want a broader change like from a greenish tone to a red one, use the gradient stripe in the middle. It also has a cursor as you can see above and reacts to drags.
So all that is fairly self-explanatory. Once you dismiss the Color Mixer, you will see that your Color Indicator has updated to the new colors. There is, however, a subtelty that needs to be addressed as well. In Quincy every cell has a color. It is the color you originally used when you drew it in Quincy's Draw mode. The subtle point is that you can change the colors of all cells that share it even after the fact, and, you don't have to be in Draw Mode to do that. In the picture above we changed the 2nd color and the grid display updated itself as you can see.
There is yet more to the Color Mixer. With the iPad in Performance View i.e. landscape rotation or on the 3.5" iPhone there is no on-screen color selector. Therefore selecting a color chicklet (button) in the Color Mixer also makes it the current draw color.
Colors play a pretty important part in Quincy, so it's good to have all this information on hand. Next we are headed over to the other end of the toolbar and take a good look at our general settings window.