This page gives some basic details about the Custom Color Render Element and how it is used in compositing.
The Custom Color Render Element is a generic render element container used for referencing other render elements of type RenderChannelColor containing color information .
||Render Settings window|| > Render Elements tab > Custom Color
The parameters for this render element appear in the Attribute Editor under Extra V-Ray Attributes.
Enable – When enabled, the render element appears in the V-Ray Virtual Frame Buffer.
Deep output – Specifies whether to include this render element in deep images.
Alias – Specifies an identifier for the render element. This can also be used to reference other render elements through their Vray Alias [vrayRE] attribute (found in Extra Attributes rollout; not the cleaner Extra VRay Attributes rollout).
Name – The text added to the end of the rendered file, when saved as a separate file (e.g. myrender.custome_color.vrimg).
Denoise – Enables the render element's denoising, provided the Denoiser render element is present.
This render element is not supported with V-Ray GPU rendering.