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Introduction
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I am relatively new to painting. As such, I'm always online looking for tutorials. However, odds are that the tutorial is being given with different paints, or a different brand than I posess. So it came down to finding a good approximation.

As well, my town is a Reaper paint deadzone. Not one hobby shop carries Reaper paint, but I really love their triad system. I needed something that I could use to spot-fill gaps in my collection without needing to order \$X of stuff to reach a shipping threshold, or paying the ungodly shipping to Canada.
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There are a variety of "colour match" spreadsheets that are available, notably from Vallejo, but I always found these to be very sparse. Only a very select few colours are listed as "matches", but 90% of the time I don't need a match for a colour, I just need to get close. So I developed some code to calculate the minimum "colour distance" and compiled the data in this spreadsheet.
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These spreadsheets should help identify colour matches for a variety of miniature paint brands. Each colour from each brand is compared using an equation called CIE2000 by the International Commission on Illumination (CIE). Then the colour with the smallest ΔE value is returned. More information on the calculation can be found here: http://en.wikipedia.org/wiki/Color_difference. First we convert an RGB colour to sRGB, then to XYZ, then to CIELAB, then we calculate the ΔE.
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CIE2000 results in a value, ΔE00, that is the "distance" of the sample colour from the reference colour. The lower the value, the better, with 0.00 being an exact match.
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To use this document, just open the page corresponding to the brand you want to reference, and find the columns corresponding to the company you want to sample. The best match across all companies is BOLDED.

For example, if I want to find which Citadel Base is the best match to P3 "Gnarls Green", I go to the P3 page and look under the Citadel Base columns. In the Gnarls Green row, I see that the nearest match is Waaagh! Flesh, with ΔE of 3.85. That's a "good match", but is approaching a "mediocre match". If I have it, I should choose to use Reaper MSP Pine Green instead.

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Details
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I made a few assumptions in creating this document.

* First, I assumed that all colour swatches provided by the various companies were accurate. To be accurate, the company that uploaded the colour sample would have had to use a calibrated monitor so that the on-screen colour matched their swatch. It does NOT require that YOU have a calibrated monitor.

* Second, I assumed that all colours used the sRGB gamut. Since most (ie 99%) of modern computer monitors are sRGB-compliant, I figured this was a safe assumption.

* Third, I assumed that all brands are capable of accurately reproducing their paint in every single batch. Minor variances may occur.

* Last, I excluded all metallic colours, washes, and inks from the calculations. It's unfair to compare an ink to a paint, or a wash to an ink, or metallic to non-metallic. I left the Reaper "Liner" paints in the calculations, because I had no idea what properties they possess. I presumed they were just paint.

* I am not responsible should you order a paint and find out that it doesn't match. Sorry, but I'm not comparing physical swatches (nor do I have perfect colour vision). I'm only doing math based on data that is widely available.
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Legend
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Colours with ΔE < 2.3 are visually indistinguishable. These are coloured blue to help identify them as "perfect matches".
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Colours with ΔE between 2.3 and 4 are sufficient matches. These colours are visually distinguishable, but they're close enough to be considered a match. These are coloured green to help identify them as "good matches".
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Colours with ΔE between 4 and 6 are mediocre matches. These colours are fairly distinguishable but you can probably still kind of consider them matches. These are coloured orange for "mediocre matches".
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Colours with ΔE between 6 and 8 are bad matches. If you use these as a match, you're really stretching the limits. For some colours, this range is already noticably different from the reference colour. These are coloured red as "bad matches".
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Colours with ΔE greater than 8 are probably not even recognizable as the same colour any more. These are coloured black as "Don't consider these to be a match".
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Final Thoughts
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I hope that you find this document as useful as I have. If you have a brand of paint that you would like included, and you have a webpage or image with colour swatches for that brand, let me know and I'll do my best to incorporate it into this spreadsheet.

Given that the delta E calculations are done in code, it should be entirely possible to build a webpage that lets a user specify a custom set of colours (eg My Paint Set) to compare against a brand as well. That way, if you can compare your personal collection WITHIN a particular brand for the purposes of tutorials (you may not have Blood Red, but Fresh Blood is going to be close enough).

I'm not nearly savvy enough to code and host a decent-looking website, but if someone wants to take the initiative, I have the code for calculating E00 available in JavaScript.
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Sources
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Reaper Master Serieshttp://www.reapermini.com/paints/corecolors
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Reaper HDhttp://www.reapermini.com/Paints/hd
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Formula P3http://privateerpress.com/files/Formula%20P3%20Paint%20Reference%20Sheet.pdf
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http://www.games-workshop.com/gws/catalog/armySubUnitCats.jsp?catId=cat840001a&rootCatGameStyle=paints-tools
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Reaper Pro Paint (discontinued)http://battlemart.com/store/index.php?_a=viewCat&catId=71
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