How do I create infinite changing background?

Hi im triying to create a game similar to the flappy bird in terms of the background, I need it to be endless but also I need it to change gradually ( similar like if you are going deeper into the ocean) I need it to become darker as you are moving foward.

Any help? thanks a lot seriously, Im getting so frustrated with this haha.

First do u have the game built? if not u can use the flappy bird template and go from their, second u can use a sprite as a trigger event to make the background change color.To do this place it in between both pipes make sure collisions are on, now create a code "on collision with other object" select the corresponding sprite your colliding with,now all u have to do is create several animations that have different colors lets say grey fading all the way to black you will also need a variable name it anything for example background switch, create different number values and set each animation to the color u want it to be also make sure the variable will go up and reset on the value u want it to, for example if background switch = 1 set animation "Default or animation name"

make sure every time u pass that sprite it adds one to the variable doing this will lchange the color then u could reset it to go back whenever u want to

thx a lot im gonna try that