Tip 3: How CMYK affects RGB
If you are supplied an image which is in the CMYK colour-space, the CMYK default may be used to interpret the intended colour appearance of the image. 

In these cases, it is beneficial to use the most recent European print standard. This is currently represented by the ‘Coated FOGRA39’ CMYK profile, and the previous ‘Coated FOGRA27’ profile will also be acceptable.

Tip 4: Profile Mismatches 
The Colour Preference settings, as recommended in Tip 2, include the option to display the ‘profile Mismatch’ warning dialogue when you open an image that uses a colour space that differs from your specified ‘Working RGB/CMYK profile’.

For consistency of your image preparation and workflow, for RGB images, choose ‘convert documents colours to the working space’. Images identified as being in CMYK mode should initially be opened choosing ‘Use the embedded profile (instead of the working space)’.

Once open, use Edit > Convert Profile to convert the image from its CMYK profile to the RGB ‘Working space’. 

Note that converting the image at the point of opening into your default CMYK colour space would be an unnecessary conversion as it ultimately needs converting to RGB. Each time an image is ‘converted’, there is loss of data.

Tip 5: RGB or CMYK?
The top bar of the image window in Photoshop indicates if the image you have opened is in RGB or CMYK. 

If it is in CMYK, you should choose Edit > Convert Profile and convert the image to your ‘Working RGB’ profile default (which will be set as sRGB for such work).

Tip 6: Optimise image depth
Photoshop’s Levels dialog (Image > Adjustment > Levels) is the best starting point for image optimisation. This displays a histogram indicating the distribution of pixel values in your image. If your image is underexposed, there will be no pixel values at the highlight (white) end of the scale.

Moving the white start point will improve the image contrast. Similarly an overexposed image will have no true black values and the shadow (black) end of the scale can be adjusted accordingly. Note that on images where there is no true white or black, a partial adjustment will be more appropriate.