Colour theory question

[Aussie iTV]

Hi there,

I realise this question isn't specific to Affinity's products, but it is related to my usage of Affinity, so I hope it's still reasonable to post here as I think the Affinity community may have someone who knows the answer/s

These questions come in relation to a previous topic I have posted here 

 

I have a 2 part question (with images attached) for someone who may know colour theory, are these formulae something that can be put into an Excel spreadsheet so that I can create a spreadsheet that will convert colours from RGB -> CMYK, and other colour conversions which I have apparent formulae for...

Are the three/four different colour codes suppose to give the exact same result in colour? I was using Affinity to change a PNG file that had black lines, and the method I used was to convert the document to a CMYK document, then use K-only to be able to change the black to any colour I picked, but while just testing out the theory, when I set it to the greens CMYK code, then changed the document back to RGB, it didnt seem to change to the colour it should represent in RGB if I understood it correctly....

 

Thanks!

Aussie

[v_kyr]

Well first of all you maybe should be aware of RGB vs CMYK color-gamut differences here, before delving into formulas.
 

Quote

 

Color Gamut
A gamut is the complete range or scope of something. In digital and print design, a color gamut is the range of colors you can achieve using different color combinations like CMYK and RGB. An RGB color gamut is much larger than a CMYK color gamut, meaning you can achieve many more colors in RGB than you can in CMYK. This is why your design may look awesome on screen, but when printed it becomes muddied and dark. Some colors can not be reproduced using CMYK so when you convert the image from RGB to CMYK, some colors will lose their vibrance.

The above graphic shows the visible spectrum we can see, the colors you can achieve using RGB, and colors achievable in CMYK.

 

 

[Aussie iTV]

7 minutes ago, v_kyr said:

Well first of all you maybe should be aware of RGB vs CMYK color-gamut differences here, before delving into formulas.
 

 

I definitely knew there was a major difference between the two, and knew that CMYK is typically used for print work, I use to work with an older gentleman who worked in lithography, and was very verse in CMYK colour spectrum.

I have too noticed that using the green of the attached image colour code, it is far duller when using the CMYK code, which while at the moment while I am just fiddling around to work out how things work isn't a big deal, but as per my other post where I was trying to change the colour of a lightning PNG (as a part of a brush set purchased on the Affinity store) the repsonse was to use K-only to alter the colour, which obviously means using CMYK to do so, when all the amature vector work I have done so far is in RGB, creating a problem for me if I desire a specific colour for the lightning PNGs but I am restricted by the limitations of the CMYK spectrum.

 

Additionally, the forumulae is so I can create a spreadsheet to help calculate conversions, while I obtained the screengrab from a site that will do it for me - I have a love of creating spreadsheets and this would be a great spreadsheet to work on if I can understand what the formulae is needing done; multiplications, power of etc... Having looked at HSL and other conversions, the formulae get even more complicated too

[lepr]

These naive conversion formulae take no account of colour profiles and their results will only confuse you when working in Afffinity.

[Aussie iTV]

1 minute ago, BofG said:

This is true. Especially as Affinity assigns profiles to every document (as it should).

Proper conversion of colours between colour spaces needs to use ICC profiles on both ends of the conversion. It's not something you will be doing in a spreadsheet.

@anon2 Have you deliberately blurred your user icon, or do I need to go to my optitions?

Damn, I was looking forward to doing a complicated spreadsheet.

What would the buzz words be to look into proper conversions with ICC profiles? Or would it just be that "rgb to cmyk conversion with icc profiles"

 

And his picture is definitely blurred to me too

[lepr]

Just now, BofG said:

 

@anon2 Have you deliberately blurred your user icon, or do I need to go to my optitions?

you need a spellchecker, not an optician, lol

[Old Bruce]

1 minute ago, anon2 said:

you need a spellchecker, not an optician, lol

Sometimes spellcheckers are enabled in the options.

[Aussie iTV]

Just now, BofG said:

How technical are you? It's a pretty deep rabbit hole....

Is it as deep of a rabbit hole as Youtube is at 2am when you just click "one more compilation video" for the 74th time?

To be honest, creating a colour conversion spreadsheet would be for the fun of it, because I'm a nutter who loves creating spreadsheets, so I'm down to grab onto Alice's hand and jump down the rabbit hole

[v_kyr]

Yes, dealing with the Affinity supplied ICC profiles is essentiell here for any color mappings. In this regard some of the LittleCMS tools and their color engine might be useful to use too. - Further tools like ColorAnt are valuable in this whole context.

[Palatino]

4 hours ago, AussieGamerGuy92 said:

To be honest, creating a colour conversion spreadsheet would be for the fun of it, because I'm a nutter who loves creating spreadsheets, so I'm down to grab onto Alice's hand and jump down the rabbit hole

The conversion is not linear. When converting RGB to CMYK, both the color properties during printing and the paper properties are taken into account. Also the color interactions between C, M, Y and K.

In practice, a CMYK profile is not calculated, but measured with colorimeters.

[Palatino]

2 minutes ago, BofG said:

The profile has already taken these properties into account. Converting between profiles doesn't require any further measurement.

This is of course correct. I wanted to say that the cmyk profile is based on a measurement and not a simple formula.