Replace Color results in muddy gray

Using Photoshop CS2 on a Windows XP Professional platform.

I have a (300dpi) CMYK phhoto image of a cross section of blades of grass set against a whitish "sky."

I need that "sky" to be a shade of green, so I tried to use Replace Color to change the "sky" color from white to a dark green (C78 M41 Y81 K35). Used the eyedropper in the Replace Color tool to select that whitish sky color, and it seems to recognize the areas I want changed, then I click on the result color and use the CMYK settings in there to set up the green that I want the selected area to become...

BUT the "result" color in the actual photo image keeps ending up as a dull muddy gray. I eyedropper that and it's not the CMYK mix I wanted at all!

Hoped to use this because of the irregular contours of the many blades of grass against that backdrop (it'd be a select/fill nightmare).

The hue of green (the actual CMYK mix) must match a solid backdrop in an area in the InDesign file into which I'm going to place the grass cross section image.

Any suggestions on why I can't get the replacement color to actually BE that green?

Works fine on my system. Do you maybe have Gamut Warning turned on?

Jan,

please publish your photo. I'm sure you'll
get many suggestions how to improve it.

Personally, I wouldn't use Replace Color
for anything else than creating masks.

Best regards --Gernot Hoffmann

Replace color only works when there is some degree of saturation, which results in an identifiable "color". Unsaturated areas -- whites, grays, and blacks -- have no "color" to replace.

You could use Select > Color Range to select the sky, then apply a Solid Color adjustment layer and play with the opacity and blending mode.