We describe a language-based approach to derivation of software product lines. A single general model, described as an I/O-alternating transition system, is used as a description of the available functionality. Hierarchically organized behavioral specifications define the actual family members by restricting input and output abilities of the general model. I/O alternating transition systems are used to model semantics of both systems and environments. Our environments are novel in that they not only restrict possible input traces, but also exhibit inabilities in distinguishing output traces. Some outputs are indistinguishable for a given environment in the same way as a color-blind person cannot distinguish some colors. Color-blindness can be used to model surprisingly many aspects of realistic environments (for example causality between firing and timing-out of a stop-watch, boolean memory flags, or use of a single actuator in place of two). The environments which are formalized as color-blind I/O-alternating transition systems, can also describe dynamic properties such as; an output that is ignored only after a certain set of other outputs.