How can I make the colours decrease in value for a smooth transition once the RGB values hit 255?

Question

int a = 0;
int b = 0;
int c = 0;
void setup () 
{
size (500, 500);
background (0);
}
void draw () {
fill (a, b, c);
circle (mouseX, mouseY, 100);
if (a<255) {
a+=1;}