accumulation buffer (color overflow)

when I add some shapes to the accumulation buffer it sums their color. when the color is greater then 1 some wierd colors are displated.
how do I limit the color to 0-1 in a way that if the color is greater then 1 I’ll get 1 (maximum allowed). the same as when I get a negative values it makes it 0.
I’ll be happy to recieve pointers to the accumulation buffer algoritm or any other explanation about the wierd colors beyond 1.