metal:set-clear-color

(metal:set-clear-color ctx rgba)

The colour a frame starts from, as an (r g b a) list. metal:attach takes the first one; this changes it afterwards. Part of the metal package, a Metal drawing surface written in rontolisp over objc and loaded on first use: macOS (java -jar, the rontolisp binary, or a compiled .class / .jar; never a .wasm), with a display. See the macOS GUI guide.

CL-USER> (metal:set-clear-color *ctx* '(0.0 0.0 0.0 1.0))
NIL