Based on the visual aspect:
* Abstract: This implies a lack of recognizable elements or composition, contrasting with the pieced-together nature of a collage.
* Solid: This suggests a single, unified surface, the opposite of the fragmented and layered look of a collage.
* Uniform: This refers to a consistent appearance, contrasting with the variety of textures and materials in a collage.
Based on the creation process:
* Original: This emphasizes a single, unique creation rather than a piece assembled from various elements.
* Single-source: This suggests a work made from a single material or using a single technique, opposite to the diverse elements of a collage.
Based on the concept:
* Simple: This emphasizes a lack of complexity, the opposite of the often-layered and multi-faceted nature of a collage.
* Unity: This emphasizes a sense of wholeness and cohesion, contrasting with the juxtaposed nature of a collage.
The best antonym will depend on the specific context and the aspect of "collage" you want to contrast.