I've seen a lot of posts from people wanting to know how to turn a colour picture into a black and white picture, but retaining some elements in colour, like this:

It's not too difficult to do, but, as so often in Photoshop, there are many different ways to achieve the same result. It's also true that there is no one best method as the most effective option  can depend on the what the image is like. So this article won't exactly be a "tip" in the classical sense of the word, but more a discussion of options.

The simplest method to use is probably the history brush. It also has the advantage that it works in 16-bit mode. Start off by saving a snapshot of the colour version of the picture by clicking on the camera icon at the bottom of the History Palette. (You could just use the original file, if you wish). Then create a Black & White version using any method you wish. In this first example, I have used Photokit "Color to B&W Set" and applied the yellow filter. Save this as a snapshot as well.

You are now ready to mix the two versions. Whether you choose to paint colour on top of B&W or B&W on top of colour is immaterial: whichever you decide, click the box to the left of the snapshot and the history brush symbol will appear. You can see it next to the Colour snapshot in the screenshot to the left.

Then select the history brush tool from the toolbox: it has the same icon that we see in the screenshot. Whichever snapshot you have decided to use for history, make sure the other snapshot is selected as the starting point for work. You can now start to paint on the image revealing the colour. If you make a mistake, you can switch the history pointer temporarily to the B&W version and fix it before continuing.