RGB v CMYK

**tweek** · 09-02-2006, 04:58 PM

People tell me RGB is for electronic-specific output like TV, Internet, etc while CMYK is used for printing. Is there any truth in it or is it user-specific preference?

**Sponsor** · 09-02-2006 04:58 PM

**chairman** · 09-02-2006, 05:06 PM

yeah there is. RGB is the color mode used by screen (monitors, tv, etc..) while CMYK is the mixture of inks used for printing. if you're designing for web, you usually design using an RGB color space. when you're designing for print you use CMYK.

**polar_bear** · 09-02-2006, 10:03 PM

Another important point is that RGB and CMYK and different color gamuts (or range, so to speak). RGB covers a wider range than CMYK, so to further Chairman's point, if you design using an RGB space and you want to print on offset and convert to CMYK, some colors will be out of range.

other points:
- CMYK has 4 channels while RGB has 3. so CMYK sizes are larger by about 30%
- RGB for web is usually handled at 72 dpi, while CMYK is usually planned for the output mechanism:
- most color separation/offset printing is handled at between 120 to 150 lpi (lines per inch)*. your working resolution would then be betwen 2 to 2.5 times the LPI (news print can be between 75 to 100 LPI, i think)

* some high-end offset (like glossy coffee table books) can be up to 300 LPI

bests,

jr

p/s numbers above, especially LPI, may have increased since the last time i looked

**tweek** · 09-02-2006, 10:20 PM

thanks folks. now i wonder why my RGB .psd changes color when I convert it to "Save to Web" .jpg under PS CS2.

**kaffee** · 09-02-2006, 11:04 PM

Originally Posted by tweek

People tell me RGB is for electronic-specific output like TV, Internet, etc while CMYK is used for printing. Is there any truth in it or is it user-specific preference?

hi,

RGB (RED / GREEN / BLUE) are the signals that are transmitted by video, hence, you are right when you say RGB is electronic-specific. The signals are split into these three 'colors' to produce what you see on the tube (TV, monitor, etc). However, these are NOT colors as in pegment (or paint, ink) that are used on paper. Therefore, when you use color for the internet, which is monitor dependent as with all other computer outputs, you use the approximate RGB color but the values are named differently (eg HEX colors).

CMYK (CYAN / MAGENTA / YELLOW / BLACK [yes, "K" is for BLACK]) are the four pigments, or inks, used in color printing. Applied using different values, or percenatges, they can produce (rather, more like 'RE-produce') color on paper.

Cheers :-)

**dominiquejames** · 09-02-2006, 11:14 PM

And, other than the Indexed and Lab colors, among others, we also have to consider if the gamut/color space you are using is sRGB or Adobe RGB.

**chairman** · 09-03-2006, 12:18 AM

Originally Posted by tweek

thanks folks. now i wonder why my RGB .psd changes color when I convert it to "Save to Web" .jpg under PS CS2.

did you calibrate your monitor/lcd? that might be the one causing the problem. you have to set your color profiles so that when you do a save as it doesn't shift colors.

**tweek** · 09-03-2006, 01:19 AM

i calibrated my screen but i think i fouled it up. i find it odd that .psd can keep one color and change colors when saved to .jpg

Originally Posted by chairman

did you calibrate your monitor/lcd? that might be the one causing the problem. you have to set your color profiles so that when you do a save as it doesn't shift colors.

**dominiquejames** · 09-03-2006, 07:03 AM

Tweek, the "Save For Web" feature automatically converts the color space of your original file from Adobe RGB space to the sRGB color space. In this case, your best bet to avoid color shift is to "Save As" JPEG instead of "Save For Web". I may be mistaken but I don't think color-calibrating your screen has something to do with the file's changing colors when saving to another format. But, you might need to re-do your color calibration just to make sure that your color judgement (for viewing purposes and adjustments) are accurate.

**chairman** · 09-03-2006, 11:55 AM

Originally Posted by dominiquejames

Tweek, the "Save For Web" feature automatically converts the color space of your original file from Adobe RGB space to the sRGB color space. In this case, your best bet to avoid color shift is to "Save As" JPEG instead of "Save For Web". I may be mistaken but I don't think color-calibrating your screen has something to do with the file's changing colors when saving to another format. But, you might need to re-do your color calibration just to make sure that your color judgement (for viewing purposes and adjustments) are accurate.

actually i think it does. from my experience it does have an effect. what i do is with whatever file i work with, i use my calibrated profile (ex. powerbook_profile vs. sRGB profile) instead of the standard sRGB. then when I do a save for the web it retains the color. Photoshop will convert the colors and approximate it to look similar. you can try that.