RGB to GRAYSCALE
It's very tempting to go down this route. Simply IMAGE...Mode...Grayscale and that's it. However unless your image is destined for black-and-white output we'd advise against this method as it throws away two-thirds of the image information by condensing three channels (RGB) into a single greyscale channel.
An advantage of converting via grayscale is that it opens up the Duotone/Tritone/Quadtone capabilities, which can generate very subtle effects.
Photoshop lets you create monotones, duotones, tritones, and quadtones. Monotones are grayscale images printed with a single, nonblack ink. Duotones, tritones, and quadtones are grayscale images printed with two, three, and four inks. In these types of images, colored inks are used to reproduce tinted grays rather than different colors. This section uses the term duotone to refer to duotones, monotones, tritones, and quadtones.
To get this options box, go to IMAGE...Mode...Duotone and click the Type box to get the drop-down menu.
Make sure the "Preview" box is checked. Adjusting the graph lines alters the image. # Adjust the duotone curve for each ink by dragging a point on the graph or by entering values for the different ink percentages.
In the curve graph, the horizontal axis moves from highlights (at the left) to shadows (at the right). Ink density increases as you move up the vertical axis. You can specify up to 13 points on the curve. When you specify two values along the curve, Adobe Photoshop calculates intermediate values. As you adjust the curve, values are automatically entered in the percentage text boxes.
However, remember that this still doesn't alter the fact that valuable color information is being thrown away. With this in mind, always create a duplicate of the original image and apply grayscale/duotone etc. effects on the copy.