Main Material Node Breakdown
The Main Material node is responsible for displaying the results of all the Material Expression nodes that are input into it the various inputs.
Each input on the Main Material node has a unique effect on how the Material will look and perform.
A White colored input means that input will have an affect on the Material, while grayed out input will have no affect on the Material.
By default, the Main Material node does have some of its inputs grayed out.
To enable inputs or disable inputs on the Main Material node, the following properties have to be modified in the Details panel of the Material.
To access the Details panel you need to make sure that you have the Main Material Node selected. To select the Main Material Node click on it with the Left Mouse Button.
When enabling a certain input, like Opacity, the newly enabled input will be displayed in the color White.
Keep in mind that enabling one input might disable another.
For example, changing the Blend Mode from Opaque to Masked will enable Opacity Mask.
While changing the Blend Mode from Opaque to Translucent will disable Opacity Mask but enable Opacity.
You can still have connections made to grayed out inputs on the Main Material node however they will do nothing.
The only time the inputs will ever affect the Material is if its inputs are enabled like in the following example.
In this example, a Material Expression node has been input into Opacity but it has no effect on the Material because the input is grayed out thus not active.
To enable the Material to make use of Opacity, change the Blend Mode from Opaque to Translucent.
This will activate the Opacity input allowing the Opacity amount to be changed via the Material Expression node that is input into the Opacity input
like in the image below.