Calculates a color value using a range pointer that lies somewhere between two colors. Originaly used an Android ProgressBar as the pointer.

ColorInterpolator.java

/*
 * Copyright (c) 2016. Al Warren.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.graphics.Color;

/**
 * Range based color interpolator
 *
 * Calculates a color value using a range pointer that lies somewhere between
 * two colors. In the original application,a ProgressBar value was used as
 * the pointer.
 */
class ColorInterpolator {
    private static int RANGE_UNIT_MAX = 100;

    /**
     * Interpolate using integer color values.
     *
     * @param startColor int
     * @param endColor int
     * @param rangePointer int
     * @return int
     */
    public int colorChange(int startColor, int endColor, int rangePointer) {
        int red, green, blue; // rgb components
        int deltaR, deltaG, deltaB; // change in rgb components

        deltaR = Color.red(endColor) - Color.red(startColor);
        deltaG = Color.green(endColor) - Color.green(startColor);
        deltaB = Color.blue(endColor) - Color.blue(startColor);

        red = Color.red(startColor) + (deltaR * rangePointer / RANGE_UNIT_MAX);
        green = Color.green(startColor) + (deltaG * rangePointer / RANGE_UNIT_MAX);
        blue = Color.blue(startColor) + (deltaB * rangePointer / RANGE_UNIT_MAX);

        return Color.rgb(red, green, blue);
    }

    // interpolate using string color values

    /**
     * Interpolate using string color values
     *
     * @param startColor string
     * @param endColor string
     * @param rangePointer int
     * @return int
     */
    public int colorChange(String startColor, String endColor, int rangePointer) {
        int startR, startG, startB, endR, endG, endB; // rgb components
        int deltaR, deltaG, deltaB, red, green, blue; // change in rgb components

        int color;
        color = Color.parseColor(startColor);
        startR = Color.red(color);
        startB = Color.blue(color);
        startG = Color.green(color);

        color = Color.parseColor(endColor);
        endR = Color.red(color);
        endB = Color.blue(color);
        endG = Color.green(color);

        deltaR = endR - startR;
        deltaG = endG - startG;
        deltaB = endB - startB;

        red = startR +  (int) Math.round(deltaR * rangePointer / RANGE_UNIT_MAX);
        green = startG +  (int) Math.round(deltaG * rangePointer / RANGE_UNIT_MAX);
        blue = startB +  (int) Math.round(deltaB * rangePointer / RANGE_UNIT_MAX);

        return Color.rgb(red, green, blue);
    }

}