Are you as Philosophers allowed to say that the rock on my desk is red? For we really don't know. We perceive it as red but what if our eyes are not showing us what is really there? For all we know, everything could be black and white.

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The possibility that the world is black and white, which you mention in your question, has been discussed by those great philosophers Calvin and Hobbes.

There are many serious questions along these lines. The redness of your rock seems to be a property the rock has as it is in itself, but early modern philosophers, beginning with Descartes but perhaps most famously Locke, questioned whether that is so. There are many sorts of alternative views, but perhaps the most common nowadays is the so-called dispositional theory of color: Colors are relational properties, on this view; for a thing to be red is for it to tend (under normal conditions) to cause certain kinds of sensations in perceivers. If that is right, then there is a way in which color is only in our minds. See, as usual, the Stanford Encyclopedia for more on this issue.

Note, however, that this issue isn't really best formulated as one about whether we know the rock is red. The issue is one about what it is for the rock to be red. On either view, we can and often do know such things. Or, at least, whether we can or do is an independent question.

The popular dispositional theory of colour that Richard mentions has a curious consequence. If being red is just being such as to tend to produce a certain kind of sensation in us, then it isn't even possible that what tends to look red to us isn't really red but is really say some shade of gray. For on the dispositional view, red just is tending to look red.

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