Both of these stamps were released for use in 1889. The very noticeable difference in the forged 3c is the very much deeper shade of colour. There is also a difference in shade with the forged 4c but it is more subtle.
The 3c stamp has a chalky surface to it which sort of washes away on soaking and so it is much more noticeable in a mint stamp. The forgery has a dull deep violet colour. The second row shows the forged stamp in various guises including an imperf and one with a good imitation of the 14 bar cancel.
It is more straight forward with the forged 3c. All the corner ornaments are not well centred and in particular the two upper ones which are centred more inwards as shown here.
Got this recently for a few pennies. Looks like the real thing and is the moderately valuable imperf between horizontal variety.