In this example, the block reference resides on an orange layer or has its color property set to "orange":
If we move the block reference to a different layer or set the color of the block reference to a fixed color (instead of "By Layer"), the entities with color attribute set to "By Block" change their color accordingly.
QCAD renders the entities using the color of layer "2" (red). QCAD looks up the layer the block reference is placed on, that's layer "2". QCAD looks up the block reference and sees that its color property is "By Layer". QCAD looks at the color property which is "By Block". The color of these entities is determined as follows: In this example, the block reference is placed on layer "2" and has its color property set to "By Layer". These entities don't inherit their color from the layer they are on ("2") but rather from the block reference they are in. They have their color property set to "By Block". The bottom triangle and text are also on layer "2". They too have their color property set to "By Layer" which means these entities also inherit their color from the layer they are on, in this case blue. The third triangle and text reside on layer "2" which is blue. The layer the block reference is placed on is irrelevant. Since layer "1" has its color set to "red", these entities are displayed in red. Their color property is set to "By Layer". The second triangle and text reside on layer "1". This means that this text and triangle will always be displayed in green. Its color property of these entities is set to "Green". The first triangle and text at the top is placed on layer "0". The block definition contains four texts and some line entities on various layers with various attributes: The block reference is placed on layer "2": ExampleĬonsider the example drawing below which consists of one single block reference. 
This situation is covered by the "By Block" attribute value available for entities. There are, however, situations in which an entity should take on the attribute of the layer on which the block has been inserted. The block might be inserted on various layers which does not (and should not) affect the attributes of the individual entities inside the block. This is important as there might be entities on various layers with various attributes in the same block. This is not only true for entities in the main drawing but also for entities that are inside a block. If an entity is on a red layer, the entity is red, if it's on a blue layer, the entity is blue, etc. In CAD, entity attributes are usually controlled by the layer an entity resides on.
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