Dhambulla - is not fully attributed to 1st C BC. There was continuous work on these and most certainly was a pure buddhist site in its early days. The additions continued as late as 9th C. the particular mural listed as 1st C Bc is definitely not pertaining to that period - based on stylistic and colors seen, it would be as late as after 10 thC CE.
The defn give away is the pedestal design he is seatedon, the styling of the mouse and ornaments etc.
If you want to study the cave mural styles then refer to the famous sigriya murals ( the famous lion fortress which Ponniyin selvar visited dressed like a elephant mahout)