Matching paint colours

Honestly if you ask five conservators how they might retouch a painting you might easily get five different answers. However one thing will always remain the same: the colors/paints that trained conservators use are stable and readily reversible. So not only will our color palette most likely differ from an artists palette, our medium tends to differ as well. We choose paints that are chemically different from the original paint (e.g we might use urea aldehyde paints on an oil painting but NEVER oil paints on an oil painting)….and I am not sure you would WANT to touch up your painting with paints that are readily reversible! Should your painting need cleaning or even re-varnishing in the future, all of the “touched up” areas would be at risk of partial or complete removal. So we do not use oils or alkyd paints to retouch artworks b/c a) they will change in their appearance over time and b) they are not readily reversible.

As for color matching….you have already cited one of the major challenges that conservators often face when dealing with retouching. And sadly there is no smoking gun here…it takes years of practice and patience to be able to balance hue, chroma, gloss/matte, and even texture. There have been some instances when conservators turn to science, using something like a colorimeter to obtain numerical data of a colored passage on a painting to help inform them when choosing a particular mixture of pigments. But unless you have one handy this is not especially feasible. There is a document on myacademia page that you are more than welcome to take a look at. It lists the relevant optical properties for specific pigments that conservators often use for inpainting.

I am sorry that we cannot be more helpful in this instance…but we will reach out to some of our color specialists on the board to see if they have any additional information that might be useful!

​Speaking in terms of what the artist might do in-studio to re-enter an in-progress or recently finished picture, the medium used in the initial work can have a significant influence on the degree of color change from wet paint to dry. A medium that helps preserve the wet appearance of colors makes it a lot easier to resume work on a dry layer. (Usually gloss mediums are better in this regard, especially ones that dry with a ‘candy’ finish.) Taking this approach can often eliminate the need to re-wet the picture (‘oiling out’).

My recommendation for sharpening the skill of mixing and matching color would be study of the Munsell color system, with color chips. Use whatever type of paint you like to duplicate the chips, alone first, then juxtaposed against neutrals and contrasting colors. (If you really want to ‘feel the burn’, use gouache.) A few weeks of this and you’ll be on your way to developing “perfect pitch” with color.

​Thanks for your insight Matthew!

As a working oil painter and someone with an interest in colour theory and colour matching, I have a couple of suggestions. Firstly I should say that almost all my experience is with oil, not oil-based alkyds, so someone else might have to address colour changes in that context. If you are touching up sections of an oil painting, it helps to know which medium was used in the original painting – best to use that as a base, if you can. If you do not know, more general criteria might apply, e.g. higher gloss and smoothness might point to the use of bodied/ stand oil in the original medium. To start matching colour, I’d suggest first oiling out the section you are touching up, and a little bit beyond (see elsewhere on MITRA for more information on oiling out. Basically apply a very thin layer of oil with a fine brush, then wipe to make the layer even thinner – just the barest minimum to get a “wet” appearance). To match the colour itself, decide whether you want to use a more transparent or a more opaque palette – let the translucency of the existing section guide you. Often you might find darker passages are more translucent and lighter ones more opaque. To select the best paints from the palette for your match can indeed take much practice, but there is a more “robotic” method similar to an approach apparently previously taught by Frank Reilly at the Art Students League (and explained to me by Dr David Briggs). Essentially it approaches the target colour by first matching value, then hue and finally chroma:
– Select two paints that “bracket” the target colour in hue (e.g. a blue-green can be bracketed by a blue and a green, an orange by a warm yellow and warm red, and so on)
– Tint or shade both paints till they are both roughly the same value as the target colour (e.g. if you are matching a light blue-green you might have to add white to both the blue and the green till they both match the target in value). With shading you have to be careful; you can use typical carbon-based blacks to shade cool colours but might have to use (raw or burnt) umbers to shade e.g. yellows. Both tinting and shading can change hue significantly, so at this stage you might find that you need to change one or both of the bracketing pair and start again.
– Next use the (tinted or shaded) bracketing colours of the same value as the target to create a mix that matches the hue of the target – start with the one already closest to the target in hue and add the other incrementally.
– If the target has very high chroma and you have chosen that highest chroma paints for matching and still can’t match it, you are out of luck – no match possible! However if the chroma of the target is lower, you next lower the chroma of your paint by mixing it with a neutral gray.
– The neutral gray should be slightly higher value than the target, if only because adding it to your mix inevitably lowers the value somewhat. Add incremental amounts of the neutral gray to your mix until the colour match is very good. To mix a good neutral is a separate topic; some manufacturers offer a range of neutral grays at various Munsell values, else you can mix your own with any white, a typical bluish black and e.g. a small amount of burnt umber to counteract the bluishness of the black.
– Some final very small adjustments might be necessary, e.g. correcting value by adding a small amount of white or correcting a drift in hue by adding small amount of the bracketing (equal value) paints.
 
You can test the colour match along the way by holding a paint mixing knife up to the section you are in-painting, or painting onto e.g. a piece of non-absorbant paper and doing the same comparison by eye or – esp. as you start getting really close – by simply painting in! If the colour does not match well, you can wipe it away; one of the pleasures of oil painting. This works best on very smooth surfaces; on a textured surface there is more risk of the “wrong” mix getting stuck in “valleys”, which you then need to remove with solvent etc  – just don’t go there – rather stick with matching by eye some distance from the painting surface.

Hope this helps!