Fireplaces, Heating Stoves, Flues and Chimneys - crumbly mortar

Last summer we got a little seaside cottage built in the '30's. It has a large river rock fireplace with crumbly powdery mortar. The stones seem secure at this point but we'd like to repair or replace the mortar. It is a light greyish color and we would prefer a dark grey/charcoal. So what to do? Do we need to remove the old stuff and then re-do it with new stuff that's already colored? Or can we just patch it up and paint all the mortar to our liking?
Many thanks!

Need a little more information here. Are you refering to the inside of the home, or the exterior of the chimney? If it is the exterior of the chimney, the crumbly mortar is a good sign of water penetration into the chimney. You'll need to have the mortar joints tuck pointed. When the repointing is done you can have the color of your chosing, or have a custom color created at extra cost.

Have your chimney inspected by a qualified professional, not just a mason. Visit http://www.csia.org to find someone in your area.