Too little magenta

[tezza]

If someone tells you that it`s 4% too little magenta in a picture. How du increase the magenta with 4%?

[DCurry]

I would help if they told you to increase the M in the midtones, shadows, or highlights - then you could target that area more precisely.

Since they didn't tell you that info, here's 2 ways:
Open up the Curves dialog (Image>Adjustment>Curves, or just press Command-M -- can also use an adjustment layer for flexibility). Select the Magenta channel from the drop down menu (says CMYK at first). Click on the diagonal line right smack dab in the middle. This adds a control point. Now, you can just type in the fields below the grid - Input: 50, Output: 54. This will affect the overall tone of the image, but more so in the midtones than in the shadows/hilites.

If you want to add a flat 4% increase of magenta across the entire image, regardless of the starting value, you could make a new layer above the image, fill it with 4% magenta, then set the layer mode to Multiply. You might get weird results in your hilites, though.

[The Repro Kid]

It is also sometimes better to remove color than to add it. If it is a color of a face you may be able to remove a little yellow rather than add a little magenta. If it is the white and black balance, again, removing some yellow will balance out the photo without adding magenta. Adding color has a way of squashing detail, whereas removing color preserves detail.

It's best to proceed the way DCurry explained until you get a little more practice with curves and color balance. Eventually you will learn to remove color rather than add it but start with DCurrys advice first.

Also, you can add an overall 4% in the first way DCurry described. Select the zero% point of the magenta curve and type in input: zero, output: 4. The select the 100% point of the magenta curve and type in, input: 96%, output 100%. This will raise your magenta by 4% on all points of the curve and will not give any weird results.

P.S. Further If you must add an excessive amount of color, an advanced way to do so without losing detail is to use selective color. But this is quite advanced and a little unintuitive. For example you would not select the magenta pop up menu in selective color to add magenta. Most likely you pick the Neutrals popup or the highlight popup or mids or shadows. You would pick just about anything but the magenta pop-up. Not very intuitive and probably very difficult to find someone to explain the proper use of selective color. But it is very powerful if you get a good background in it's use. It is a drum scanning technique and I doubt you'll be able to find any worthwhile info in any books. If you know a drum scanner operator watch them use selective color.

For now just throwing in 5 percent extra magenta at the 50% point on the magenta curve is your best bet.