In Scratch you can sense when a sprite touches a colour. It is simple and then the sprite does something.
Drag over your green flag, change backdrop and all that,
Drag over the ‘forever’ loop,
Bring the ‘if <>’ loop into the ‘forever’ loop,
Go into the Sensing section and drag the ‘touching colour <>’ into the ‘if <>’ block,
Click the coloured square in the ‘touching colour <>’ block,
Click any colour on the Scratch interface to change the colour,
Add whatever you want the sprite to do into the loop,
Click the green flag!