Most modern digital monitors use up to 24-bit of information for each pixel to display an image. Nevertheless, some multi-display systems have to sacrifice the bit-depth to support connections from tens or hundreds of screens because limited numbers of pins are available in the central processing chip. These reductions on bit-depth of color will definitely degrade the quality of the display. To solve this issue, we present a spatiotemporal dithering method which is able to extend the color depth and improve the display quality. The key idea in the proposed method is to utilize frame rate control in spatial dithering which is Floyd-Steinberg dithering in our case. This spatiotemporal dithering method takes advantage of both spatial and temporal dithering, bringing in an extension on the color depth and an improvement on the display quality without flicker problems. After implementing the hardware circuits for a multidisplay system on a FPGA device, we find that our spatiotemporal dithering method is efficient and effective, outperforming other spatial methods under the circumstances of limited color depth.