Convert images to grayscale by setting the effect parameter to grayscale (e_grayscale in URLs). You can add further transformations in order to achieve the exact result you need.

Here's an original image for example:

Here's how to apply the 'grayscale' effect:

You can also apply the grayscale effect on overlay images. For example, here's Cloudinary's logo:

We'll add it as a grayscaled overlay and make it half transparent by setting the opacity parameter to 50 (o_50 in URLs):

Here's a more advanced and interesting example of applying the grayscale effect on overlays. This time, we'll use the dog's image as a texture to the logo (Using the same technique as the recipe - Use images as text textures):

Now we'll apply the grayscale effect: Finally, we'll add the original image as an 'underlay' to get the following cool result:

Note that you can use a similar practice to grayscale only certain parts of the image:

Here's another example of using the gradient_fade effect (e_gradient in URLs) to create a smooth transition from grayscale to color: