The wall is white on the inside and green on the outside.
Literal
Wall [topic-は] inside [subject-が] white and outside [topic-は] green is.
A double-subject structure: 壁は sets the overall topic, then 内側が and 外側は provide two sub-topics with their respective descriptions. The て-form of い-adjective 白くて chains with the next clause. The は on 外側 is contrastive — 'the outside, in contrast, is green.'