What (feasible) augmented senses would be useful or interesting?

Experiments with squirrel monkeys suggests that the monkeys can effectively learn new color qualia. It requires gene therapy that's a bit more risky then brewing your own vaccine together but it's technology that works.

And most importantly, if color is a property of brains rather than something in the external world, does that imply the number of "possible colors" is infinite? 

I don't see how that changes anything about that. If you use real numbers to model color you already have infinitive shades of colors. Adding the number of possible cone configurations in the eye adds a lot of additional one's but there's a physical limit of ways those can be configured.

I don't mean discerning ever finer gradations in existing colors, but entirely new color qualia.

Finer gradations are boring, but there are (1) infrared and ultraviolet colors, and (2) colors that our current system of three types of cone cells perceives as the same, but could be perceived as different by having more types of cones, or even different types of cones. We could use the new qualia for these.

Imagine an ultra-intelligent tribe of congenitally blind extraterrestrials. Their ignorance of vision and visual concepts is not explicitly represented in their conceptual scheme. To members of this hypothetical species, visual experiences wouldn’t be information-bearing any more than a chaotic drug-induced eruption of bat-like echolocatory experiences would be information-bearing to us. Such modes of experience have never been recruited to play a sensory or signaling function. At any rate, some time during the history of this imaginary species, one of the tribe discovers a drug that alters his neurochemistry. The drug doesn’t just distort his normal senses and sense of self. It triggers what we would call visual experiences: vivid, chaotic in texture and weirder than anything the drug-taker had ever imagined. What can the drug-intoxicated subject do to communicate his disturbing new categories of experiences to his tribe’s scientific elite? If he simply says that the experiences are “ineffable”, then the sceptics will scorn such mysticism and obscurantism. If he speaks metaphorically, and expresses himself using words from the conceptual scheme grounded in the dominant sensory modality of his species, then he’ll probably babble delirious nonsense. Perhaps he’ll start talking about messages from the gods or whatever. Critically, the drug user lacks the necessary primitive terms to communicate his experiences, let alone a theoretical understanding of what’s happening. Perhaps he can attempt to construct a rudimentary private language. Yet its terms lack public “criteria of use”, so his tribe’s quasi-Wittgensteinian philosophers will invoke the (Anti-)Private Language Argument to explain why it’s meaningless. Understandably, the knowledge elite are unimpressed by the drug-disturbed user’s claims of making a profound discovery. They can exhaustively model the behaviour of the stuff of the physical world with the equations of their scientific theories, and their formal models of mind are computationally adequate. The drug taker sounds psychotic. Yet from our perspective, we can say the alien psychonaut has indeed stumbled on a profound discovery, even though he has scarcely glimpsed its implications: the raw materials of what we would call the visual world in all its glory.

Interview with David Pearce with the H+ magazine, 2009

Or, in other words: I hear your "new colors" and raise you new qualia varieties that are as different from sight and taste as sight and taste are from each other.