Re: Paint color changes

There are several things that can cause this:

1. Incomplete mixing of either the original paint or the thinned paint. The only acrylic paint that benefits from thinning for brush painting is Tamiya, and the dilution rate is very small. I usually just dip the brush in 90% isopropyl before starting and periodically thereafter. If you thinned substantially, then pigment separation can quickly become a problem.
2. Using a marginally compatible thinner. This can alter the interaction of the binder and the pigment, the behavior or the binder, or the behavior of the pigment, because it alters the chemical properties of one or the other.
3. Viscosity changes as the paint is applied. This is usually the result of going over a previously painted but incompletely cured surface. Be sure one coat has cured completly before recoating.
4. Varying thickness of the paint. The color of paint is not produced only at the surface, but through the full thickness of the paint layer. Some colors are more susceptible than others, and gloss paints are most susceptible to this. Gray colors, particularly light to intermediate shades, and those that contain larger amounts of red or blue pigments are more susceptible because they contain pigments with low opacity and/or high internal reflection.
5. Paints with high amounts of opaque but colorless pigments (called hiding agents) to improve hide (especially whites and light grays) are particularly vulnerable to shade variation when applied by brush. This effect is least obvious in whites, but often quite pronounced in grays. Something similar occurs in black, since the most common (and least expensive) black pigment is platelike in particle shape.

Triarius wrote:

Using a marginally compatible thinner. This can alter the interaction of the binder and the pigment, the behavior or the binder, or the behavior of the pigment, because it alters the chemical properties of one or the other.

Absolutely.. I have an ancient bottle of what used to be Tamiya Sky Grey, which I thinned (in the bottle, before I knew better) some 20-something years ago with methylated spirits (ethyl alcohol). It's still usable as a paint except that it's no longer sky grey, but an odd pale pink.

Another thing to consider when brush painting is that you may have some cross-contamination of another colour, due to incomplete cleaning of your paintbrush.

Another of Tamiya's paints to be wary of when brush painting is, ironically, one of the most used colours, Olive Drab. Tamiya's Olive Drab contains (among others) two prominent pigments, a deep green and a brown. These pigments tend to separate very readily and it's not unusual when making repeated brush strokes on one area, to have the green separate from the relatively heavier brown pigment and leave that area with a distinct brown cast. 

Oh, I see..thanks alot. Hmmm...I guess it should be the different number of coats that caused the discrepancy.