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

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'