Okay, so you have carved your concrete to resemble a rock and the next step is to transform it from battleship gray to something that is alive and breathing. At this point reference is super important. Sometimes you may be challenged to match "real" existing native rock found on the property or the client has strict parameters for you to follow, in any case, color observation is key!
You will find in nature that most color compositions/schemes for rock are analogous, which means that the colors are tightly placed next to each other on the color wheel. This makes for a harmonious rhythm of color play.
Lastly, color is relative! Let say a real rock-scape contains an iron oxide rust streak that may appear loud and vibrant, but one will have to consider the color surrounding it. If the streak is overlapped over a tan/light khaki, the streak will appear to be more saturated. I have attached a picture showing this phenomenon to make better sense of this concept. Notice the same blue square appears to be a different color when overlapped onto a saturated color.
I will be producing a video going more in depth on this topic and will be providing some fantastic color recipes in the "PRO" blog area for subscribing members.
Happy Carving and Painting!