Color aimbots are insanely inefficient. You'll need to write your own algorithm for quick-scanning a whole screen full of pixel using the GetPixel API/BMP method.
On my screen this would take 1366 * 768 iterations to scan EVERY pixel ONCE = over 1 million iterations. If you assume just an API call to be between 0.5ms to 1ms, you're looking at 500k-1M ms to scan the screen once without doing any processing, it's a bad idea without some form of algorithm to narrow down your iterations.