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!