In order to connect a Keyboard, Mouse and VGA color you, need a Y-Cable. You can find original ones on eBay from time to time, you can buy a new version on eBay from a gentleman that makes them, or you can build your own if you buy the parts.

I have done all three at one time or another. I put together the following diagram for those industrious souls who just want to build their own.

Link to eBay seller who makes Y-cables

Custom NeXTstation Color to Soundbox to VGA Monitor Cable