Tamiya Hull Red has a violet hue, which I don't like. To reduce the hue, what color should I add to Hull Red? Would adding Deck Tan or Buff reduce the violet hue?

Buff would be all right, but why don't you just straight for it and use a bit of dark yellow instead?

Kevin's right.

It might help to understand why:

If you perceive a violet hue in the Hull Red, then the cure is to add yellow.

It's a color-wheel problem. Red, Yellow, and Blue are the three primary colors. Red and Blue make Violet, a secondary color. Even if the Hull Red is strongly red, any violet hue you perceive must be caused by some amount of blue. (Unless the problem is the lighting that you're viewing the color under, but still...)

So, the only other color that you can add is yellow. Note, though that the result will be a "browning" of the Hull Red, unless you add a lot of yellow, which will then turn it orange. Brown is a tertiary color, made of red, blue and yellow.

So, adding a small amount of yellow to the Hull Red will change the violet hue to a brown hue. Note that the Tamiya Dark Yellow has a green cast that's caused by some amount of blue in it (yellow + blue = green). However, the green will combine with the red in the Hull Red and still create brown.

The risk is that you will add too much yellow and make Hull Orange rather than dark Hull Red, so you must be careful in how much Dark Yellow you use.

If you want a brighter red, you will have to select another red color other than the Hull Red.

IMO, Deck Tan has a grayish cast and Buff has a tannish cast. The gray in the Deck Tan might be because of a small amount of blue rather than black (most blacks are actually either "bluish" or "brownish). The tan in the Buff is caused by some red in it.

Of these two choices, the Buff would be better to lighten up the Hull Red since the Deck Tan might make the violet hue more pronounced to your eye; the Buff will lighten it but with a tan cast. The tan cast is usually perceived as dirt and wear.

You could also add a very small amount of the Dark Yellow and then add Buff or Deck Tan to lighten that color for scale effect and weathering. In this case the result would probably be: Buff = a lighter, but tanner color; Deck Tan = a lighter, but grayer color.

A lot of this also depends on your own color perceptions and the lighting that you're viewing the samples under.

You can get an inexpensive color wheel where art supplies are sold. I keep one on my painting bench as well as another graphic representation of hue, tone and value pinned to the wall. Sometimes when I have a color-conundrum, sitting back and thinking through the problem is helped by referring to these aids.

HTH,

IMO, Hull Red, when sprayed lightly, gives tank tracks a mildly rusty look. But its violet hue is not something I love.