To prepare green spread, combine spinach, mint chutney and mayonnaise in a bowl and mix well. Add salt and crushed peppercorns and mix well.
To prepare red spread, grate boiled beetroot in another bowl. Add tomato ketchup and mix. Add salt and crushed peppercorns and mix. Add mayonnaise and mix well.
To prepare yellow spread, combine crushed corn kernels, mayonnaise and mustard paste in another bowl and mix well.
Put some green spread on 8 bread slices. Similarly, put some yellow spread and red spread on 8 bread slices each.
Pile 1 red bread slice, 1 yellow bread slice, 1 green bread slice, 1 red bread slice, 1 yellow bread slice and 1 green bread slice on top of each other and cover with the remaining plain bread slice. Similarly, prepare the remaining sandwiches.
Trim the edges of the sandwiches using a serrated knife. Cut into triangles.