Moved here, because Programming Questions is reserved to questions showing some code to work on...
The classical solution is to use a secondary variable holding the previous value of your variable (that's the value that get bigger, not the variable itself!).

An example:

1. void setup()
2. {
3.   size(800, 800);
4.   frameRate(5); // Slow motion
5.   background(255);
6. }
7.  
8. float x = 100;
9. float px = x;
10.  
11. void draw()
12. {
13.   fill(255, 10);
14.   rect(0, 0, width, height);
15.   float rnd = random(-15, 15);
16.   // Variable value can increase or decreases
17.   x += rnd;
18.   if (x > px)
19.   {
20.     stroke(#FF0000);
21.     line(x, 0, x, height);
22.   }
23.   else
24.   {
25.     stroke(#00FF00, 100);
26.     line(x, 0, x, height);
27.   }
28.   px = x;
29. }