Looks Block in Scratch

Looks block are color-coded purple, and are used to control a sprite’s appearance.

There are 22 Looks blocks: 18 Stack blocks and 4 Reporter blocks. 14 are only for sprites, and 4 are only for the Stage.

 

NameBlockFunction
Switch CostumeThe block changes its sprite's costume to a specified one.
Switch BackdropThe block changes the Stage's backdrop to the specified one.

Next CostumeThe block changes its sprite's costume to the next one in the costumes pane, but if the current costume is the last in the list, the block will loop to the first.
Next BackdropThe block changes the backdrop to the next in the list of backdrops, but if the current backdrop is the last in the list, the block will loop to the first.
Say for SecondsThe block displays a speech bubble with the specified text for the sprite that runs it, which appears on the screen for the specified amount of seconds.
SayThe block gives its sprite a speech bubble with the specified text — the speech bubble stays until another speech or thought block is activated, or the stop sign is pressed.
Think for secondsThe block give its sprite a thought bubble with the specified text, which stays for the specified amount of seconds.
ThinkThe block gives its sprite a thought bubble with the specified text. The thought bubble stays until a speech or thought block with its text block empty is activated, or the stop sign is pressed.
Change EffectThe block changes the specified effect on its sprite by the specified amount. There are seven different effects to choose from: colour, fisheye, whirl, pixelate, mosaic, brightness and ghost.
Set EffectThe block sets the specified effect on its sprite to the specified amount
Clear Graphic EffectThis block resets all 7 graphic effects
Change Size byThe block changes its sprite's size by the specified amount. The default sprite size is 100; size values below that percentage are for shrunken sprites, and size values above it are for overlarge sprites.
Set SizeThe block sets its sprite's size to the specified amount. The default sprite size is 100%; anything lower than that will decrease the size of the sprite on the stage, and anything above will increase the size of the sprite on the stage.
ShowIf the block's sprite is hidden, it will show the sprite. If the sprite is already showing, nothing will change.
HideIf the block's sprite is shown, it will hide the sprite. If the sprite is already hidden, nothing happens.
Go to FrontThe block will place a sprite in front of all other sprites. It does this by changing the sprite's layer value.
Go Back LayersThe block changes its sprite's layer value by the specified amount. This block is rather unusual compared to other blocks. It moves the layer value back, and not forward. To move it forward instead, a negative number can be used.
Costume NumberThe block holds its sprite's current costume number.
Backdrop NumberThe block hold the current backdrop number.
Backdrop NameThe block holds the current backdrop name.
SizeThe block holds its sprite's size. This block can be displayed as a Stage monitor.

The above information is referenced from Scratch Wiki.