how do I do this in Gimp?

[X_lolol_X]

for example:

 

 

 

 

 

 

 

if a color is a dark shade of red...

 

 

 

and i try drawing on it with a normal white color, how do i make the white turn to a darker shade of it as well?

 

 

 

 

 

 

 

sorry if its worded bad but i couldnt think of a way to describe it :?