Clearly what is dependent on the existence of mental processes is not therefore illusory. Unlike many things, the existence of mental processes is dependent on the existence of mental processes. It does not follow that mental processes are illusory. But I think it should be said that there is little or nothing that philosophers have provided that could help us to understand what it means to say that a colour, the colour pink, for example, exists, or exists only, in the mind, or outside of it, for that matter. The mantis shrimp has at least eight colour receptors - it can also perceive circularly polarized light and its direction of rotation! - but the poor old thing does not have an awful lot in the way of a mind. Mind you, it can deliver a punch (with one hand!) at 10,000 g, enough to boil the water in front of it.
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I've read a few philosophers write that color theorists typically divide into two camps of those who say colors are scientific properties of objects and those who say that color only exists in the mind and are therefor subjective and illusory. These philosophers often offer some alternative to this dichotomy like the idea that colors should be thought of in more relational terms between subject and object. But can't colors be entirely mental or dependent on mental processes but not be illusory?