Parts with two colours

[Philippe Hurbain]

Generally, there are some parts with more than one colour, mini antenna (design 4592) for example.
Here are allways special parts needed?

Yes. But the antenna is not a good example, since it is made of two parts, the base (4592) and the stick (4593). The bicolor shortcut with black stick (4592c01) is only a convenience.
True bicolor parts (often co-injection) examples are 61406p03 (Plate 1 x  2 with Tooth and Flexible Orange Tip) or 16001p04 (Minifig Arm Left with Black Lower Arm Pattern). And in this case they are treated as patterned parts.