To add to this, the amount of energy actually being sent out of your screen isn't a linear function of the RGB values. So to find what the mixture will look lik

To add to this, the amount of energy actually being sent out of your screen isn't a linear function of the RGB values. So to find what the mixture will look like you have to convert them to linear values using the formula here https://en.wikipedia.org/wiki/SRGB https://en.wikipedia.org/wiki/SRGB, then take the average, then convert back. So '50% gray' is actually #979797. This should match the level of grey which the image appears to be if you view it at full resolution. It will only match the zoomed out image if your computer also uses the correct averaging formula.