I think these would all be white, black, or transparent, depending on what it was designed to do. It could also be a mirror for visible light and a lens for UV. An object's color is determined by what light it reflects, not what light it absorbs. In other words, a green shirt is, in one sense of the word, every color but green.
Something that reflects all colors of light is white. If an object only absorbed infrared, only ultraviolet, or only infrared and ultraviolet but nothing in-between, it would appear white, because white light is a combination of all visible colors. For reference, see these flowers:
However, insects like bees can see ultraviolet, and see these flowers completely differently:
Obviously, a bee probably sees it a little bit differently, given that our brains can't even process ultraviolet*, but this is a translation. The flowers look white to us, but they are actually creating interesting patterns for those who can see UV. I'm not entirely certain, but I think this is because certain parts of the flower reflect and other parts absorb UV, creating patterns that humans can't even see.
Now, if something only reflected ultraviolet/infrared, which I think is what you're asking about – a flower colored such that if we could see ultraviolet, we would point at the flower and call it ultraviolet – it would look black. Our hypothetical flower would absorb all colors of light and would not reflect any. This is what black is: it absorbs every visible color of light. If it allowed all other light to pass through it, it would look transparent.
Water is an example of something that reflects UV light. This is why water can actually increase your sunburn. There are also mirrors and lenses in fiber optics specifically designed to allow most light to pass through but to be opaque to UV.
*Some people can see into the UV spectrum after cataracts surgery. UV can damage your eyesight, so our eyes are designed to reflect it. After you remove that protective coating, you can sometimes see it. There is also some evidence that some people may be tetrachromats, with cones to see UV.