How to optimize query based on explain analyze in PostgreSQL?
You need this index to makenthe query fast:

CREATE INDEX ON mytable (color_code, shade_type);


If you want to aim for an index only scan, you can use

CREATE INDEX ON mytable (color_code, shade_type) INCLUDE (amount);


But that is only useful if the table gets vacuumed often enough, and you don't need HOT updates for the amount column.