Computers are notoriously fickle with colours, as different monitors, operating systems, and browsers can handle them very differently - and that's before you get into issues like monitor calibration (which most people never bother with) and lighting conditions. On my own monitor here at home it does display as a dark green (and the yarn is indeed a dark green) and the monitors in the office we used to sample & define the colours were calibrated as carefully as we could. But I suspect this may be difficult to resolve perfectly. The only real answer is for anyone considering commissioning a piece to request yarn samples in advance if they are concerned about exact shades, just as we'd recommend ordering a swatch for an off-the-shelf fabric.