This will usually require a photo editing program (such as Photoshop, or Paint Shop Pro. For a free one, try GIMP). Some cameras have a feature to do that, while others don't, but the fastest way is to use a computer.
Basically, open up the picture, and open the layer palette. Your picture should be the Background, or Base, Layer. Duplicate this layer, and then go to Layer>Adjustments>Hue/Saturation/Lightness OR Color>Adjustments>Hue/Saturation/Lightness. Then, slide the saturation bar to 0, and press okay. Now, take your eraser brush and erase the areas you want color.
Another way is to open up your picture, and add a new layer. Fill the new layer with either white or black, and then set the layer blend mode to Color or Saturation. Everything should be in greyscale. Now, take your eraser and erase the area(s) you want in color.
Hope that helped!
Another way to complete this in Photoshop or the Gimp is to use a selection tool. The magic lasso works well for areas that are not polygonal or circular. Select what you want to keep in color, then click Select>Invert. At this point, everything that you want black and white should be selected. Next go to Image>Adjustments>Hue/Saturation and bring the saturation slider all the way to the left.
.... go to this site, it will give you a program for under $20.
A step-by-step easy-to-understand illustrated example of this technique using Photoshop Elements: http://www.photoxels.com/digital--tutorials/tutorial-color-accent/
And still ANOTHER way requires two layers. This is good for photos where you want one thing to be in full color, and everything else to be b&w.
Bring up your image, open the layers palette and duplicate the layer with the image on it. Convert the file to CMYK mode, and hide the top layer. Choose the bottom layer, select all, cut, then switch to the Channels palette. Select the black channel only and paste. This creates a grayscale image.
Now, hide the bottom layer and show the top one. Use whatever selection tool you like to select the thing you want to keep, then invert the selection, feather the selection, and delete. Feathering gives a slightly soft edge, which looks good. Now, you have a colored thing with a feathered edge floating in a sea of nothingness.
Finally, show the top layer and flatten the image. If you're going to use the file online, convert to RGB.
The colors teal and purple make a blueish gray color.
black and white adding another color will make grey with a shade of that color but B&W will be neutral gray.
gray and orange will make an ugly muddy color
White and gray together will make a lighter gray. Red and green will make a brown. Light gray and brown together will make a neutral color, also called greige or a greyed brown.
Blue gray and brown would make a grayed brown color. It would be a very muddy neutral. The blue would make it a cool color.
really dark gray or light black
Gray is made by mixing black and white.
brownish, bluish colour
Gray and purple mixed together will make a less saturated shade of purple - that just means it is closer to gray than purple.
mix black with white = Grey
You will want to combine equal amounts of all colors from the color wheel. This will make a neutral gray color. You may have to practice it a few times before getting a good gray.
gray with a little white and some sparkly gray sprinkles.
The only way you can make it gray with food coloring is if the substances you are making the volcano out of is white. You will have to add black food coloring drop by drop until you have the color gray you want. If you can't find black food coloring, mix every color you have together then drip it in. All of the colors mixed together will make black.
White & a little black.
None--- Gray is a value, not a color/hue.
you can't unless if you add black
If you mix together gray and yellow the result it called a tone of yellow. It is predominately yellow, but the color is less intense. This is a difficult shade to make well because the gray can easily overpower the yellow.
Depending on how much of each color it varies from Brown,Bluish-green,Gray,or dark gray.
Mustard is a dark yellow color. You can make it by adding a little gray to a lot of yellow. Instead of gray, you may want to add a bit of brown.
Mix white and black together to make the color gris or gray. White should make up most of the mixture at about 70% to get a consistent color.
I don't knowfsdhyipink colored grey...
Indium is a metal, solid, natural, with a gray color.
Opposite colors on the color wheel should give you grayish results such as mixing red and green. You can add a touch of blue to achieve a bluish gray or a tad bit of yellow to make a brownish gray.
Brown is not a part of the color gray. You need white and black to mix gray. If you have brown you can only use it to make different shades of brown.