I love the outfit she wears in the Magic Shop for the episode Family in season 5. Amber looks amazing in that rich pea green, and the lower half really plays into her witchy side.
Most of Tara's costume choices seem to be rooted in making her like an earth goddess, though, long before Willow had her moment in the last episode. People see the costume choices as Tara being made homely or frumpy, but I see it as she , more so than Willow, is connected to all things natural and warm. Willow may be the powerhouse of duo, but she's very analytical, and in later seasons, easily disassociates herself from feelings and emotions for the purposes of achieving something. For instance, I don't see Tara killing Bambi for the resurrection spell, and Willow glosses over this aspect of the deed in front of Tara. Tara is nature. Willow is intellect. Tara is warm. Willow can be stone cold.
Willow is also often shown being introduced to a spell through Tara, only to improve on it later. Tara is where the source naturally flows from, but Willow is the one who can punch it up and make it sparkle, just like the Tinkerbell light spell. It is also worth noting that the nurturer of the pair is Tara, as she often plays surrogate mom to Dawn, and it is she who tries to ground Willow's addiction before it really gets out of hand. To this end, in season 6, we often see Tara wearing drapey skirts while Willow sports streamlined jeans. It's feminine and masculine energy contrasted. Tara is Mother Earth, while Willow comes across as Jovian.
To achieve all of this through costume, we see Tara often wearing weedy looking clothing, kind of hippy chic, but almost always muted, as though everything were plant dyed. Browns featured a lot in season 6, but we were supposed to see contrasts between her and Willow more sharply that season. Willow is often in black and deeply saturated colors, while Tara is in earth tones.
The only time I can really think of this deviating outstandingly is in the episode Family, where we are meant to question whether or not Tara is a demon. Here, her clothing looks closer to what Willow wears in season 6, but still with that splash of green, again very earthy.