Combine gradient layers - [C#]

Question

I have a program where I can make an ellipse gradient such as this:

But that's not enough for me I need more than 1. So when I try to make for example 3 gradients, and they collide with each other this is the result:

But I expect this (made in photoshop)

Clearly, something is wrong with the way I merge the layers there.

Here is the source code.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Drawing;
using System.Linq;
using System.Text;

namespace Gradients {

    static class Gradient {

        public static Bitmap GetGradient() {
            Random random = new Random();
            Bitmap image = new Bitmap(300, 300);
            int size = 300;

            double[,] colorValue = new double[300, 300];

            int amount = 3;

            for (int x = 0; x < size; x++) {
                for (int y = 0; y < size; y++) {
                    colorValue[x, y] = 1;
                }
            }
            for (int i = 0; i < amount; i++) {

                int centerX = random.Next(size / 10 - 1, size - (size / 10 - 1));
                int centerY = random.Next(size / 10 - 1, size - (size / 10 - 1));

                for (int x = 0; x < size; x++) {
                    for (int y = 0; y < size; y++) {

                        double distanceX = (centerX - x) * (centerX - x);
                        double distanceY = (centerY - y) * (centerY - y);

                        double distanceToCenter = Math.Sqrt(distanceX + distanceY) * 5;

                        distanceToCenter /= size;
                        if (distanceToCenter > 1)
                            distanceToCenter = 1;
                        if (distanceToCenter < 0)
                            distanceToCenter = 0;

                        double number = 1 - distanceToCenter; // inverting number range from 1-0 to 0-1
                        colorValue[x, y] -= number;
                        if (colorValue[x, y] < 0)
                            colorValue[x,y] = 0;
                    }
                }
            }


            for (int x = 0; x < size; x++) {
                for (int y = 0; y < size; y++) {
                    int cValue = Convert.ToInt32(colorValue[x, y] * 255);
                    image.SetPixel(x, y, Color.FromArgb(cValue, 0, 0, 0));
                }
            }

            return image;
        }

        // euclidean distance
        private static double getDistance(int x1, int y1, int x2, int y2) {
            return Math.Sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
        }
    }
}