If "speckled" means "having spots or marks":
* Solid
* Uniform
* Plain
* Unblemished
* Smooth
* Even
If "speckled" refers to a pattern of small spots:
* Striped
* Checkered
* Solid
If "speckled" describes something with a scattering of small things:
* Dense
* Packed
* Crowded
To give you the most accurate antonym, please provide me with the sentence or context in which "speckled" is used.