Even in Photoshop, you can't convert an image from RGB to CMYK and then back again without losing or altering some of the color information. EDIT: You mention a couple of times in your comments that you just want "RGB values in percentage." RGB is additive. The more color (made of light) you add, the closer you get to white. CMYK is subtractive. The more color (made of ink, which is reflective, which subtracts light) you add, the closer you get to black (or actually a muddy brown). CMYK has a smaller range, or gamut, of colors it can reproduce than RGB does. What you can do about it: To take color matching in your hands, design within the CMYK color space and then convert to RGB yourself, without losing any colors. Or, if your software doesn’t support the CMYK color space, use an online RGB to CMYK converter to check which colors of your design are out of CMYK color space and adjust those. Put your CMYK images directly into InDesign or whatever program you'll use to create your PDF. If your image is in an RGB color space (including sRGB), then use Adobe Photoshop to convert your images to CMYK following the steps below. Photoshop is a fully color-managed application and can convert your images with a minimal amount of color shift. UiQxt3.