Sandstone QuickRef, the fastest way to determine how a given RGB triplet will look once printed.
Each "wall" surface shows a 8*8 combination of 2 channels with the third at constant level. Each "room" has 3 of these walls, so you can see RGc, RcB and cGB combinations, with the contant "c" as shown in the edges.
Plus: RGB and CMYK (as defined by RGB data) gradients, to check smooth transitions; Grey steps, to check color balance; and Resolution tests combining two greys (black and white, then darkest grey with lightest grey, then mid dark and mid light and finally the two "central" greys) at 4, 8, 16, 20 and 24 lines per 3mm (~1/8 inch).
The values used to define the colours are 0x00, 0x24, 0x48, 0x6D, 0x91, 0xB6, 0xDA and 0xFF. You can download this image to use a color picker: