CREATE TABLE table_57201 ("Type of sign" CLOB, "Shape" CLOB, "Border" CLOB, "Background colour" CLOB, "Text/Symbol" CLOB)

Which sign had a yellow background and a triangular shape?
SELECT "Border" FROM table_57201 WHERE "Background colour" = 'yellow' AND "Shape" = 'triangular'
SELECT "Border" FROM "table_57201" WHERE "Background colour" = 'yellow' AND "Shape" = 'triangular'