I just had a flagstone patio done over existing exposed aggregate. They poured the cement individually for each stone they laid. I didn't want regular gray colored grout; I wanted a warmer color that would compliment the warm tones of the stones. There is a color additive (powder form) that you can buy to mix into your grout mixture. Only three choices: tan, terra cotta or black. I chose the tan and I believe the basic "grout" was regular cement. When it dried, it was very sandy in texture. You can always ask someone at your local home improvement center what to use to grout flagstone.

Use thinset mortar to set the stone. I prefer the kind used for marble in outdoor applications. It retains flexibility over time, so it can respond to hot/cold cycles. Make sure the substrate is clean, and consider using a bonding agent. If the stone is porous you should seal them before grouting. Use a quality sanded grout with some bonding agent admix (unless proscribed) in any of the many colors available.

You should pay special attention to the type of concrete used. Visit www.menaisandsoil.com.au for more details on concreting in general