ColorUtils.java

public class ColorUtils {

    private static final float DEFAULT_LIGHTNESS_ADJUST_PERCENTAGE = 0.2f;
    
    public static boolean isFlagshipLight(@ColorInt int color) {
        return getBrightness(color) >= 204.0f;
//        return getLightness(color) > 0.5f;
//        return getContrast(color, Color.BLACK) > 4.5;
    }

    public static int lightenOrDarken(int color) {
        return lightenOrDarken(color, DEFAULT_LIGHTNESS_ADJUST_PERCENTAGE);
    }
    
    public static int lightenOrDarken(int color, float percentage) {
        if (getLightness(color) > 0.5f) {
            return darken(color, percentage);
        } else {
            return lighten(color, percentage);
        }
    }
    
    /**
     * Formula defined
     * <a href="https://www.w3.org/WAI/ER/WD-AERT/#color-contrast">here</a>
     */
    private static float getBrightness(@ColorInt int color) {
        int r = Color.red(color);
        int g = Color.green(color);
        int b = Color.blue(color);

        return ((r * 299) + (g * 587) + (b * 114)) / 1000.0f;
    }

    private static float getLightness(@ColorInt int color) {
        float[] hsl = new float[3];
        ColorUtils.colorToHSL(color, hsl);
        return hsl[2];
    }

    private static int lighten(@ColorInt int color, float percentage) {
        float[] hsl = new float[3];
        ColorUtils.colorToHSL(color, hsl);
        hsl[2] = constrain(hsl[2] + percentage, 0f, 1f);
        return ColorUtils.HSLToColor(hsl);
    }

    private static int darken(@ColorInt int color, float percentage) {
        float[] hsl = new float[3];
        ColorUtils.colorToHSL(color, hsl);
        hsl[2] = constrain(hsl[2] - percentage, 0f, 1f);
        return ColorUtils.HSLToColor(hsl);
    }
    
    private static float constrain(float amount, float low, float high) {
        return amount < low ? low : (amount > high ? high : amount);
    }
    
    /**
     * WCAG 2.0
     * level AA requires a contrast ratio of 4.5:1 for normal text and 3:1 for large text.
     * Level AAA requires a contrast ratio of 7:1 for normal text and 4.5:1 for large text.
     *
     * Large text is defined as 14 point (typically 18.66px) and bold or larger, or 18 point (typically 24px) or larger.
     *
     * <a href="http://webaim.org/resources/contrastchecker">reference</a>
     */
    private static double getContrast(@ColorInt int foreground, @ColorInt int background) {
        // [1:21]
        return ColorUtils.calculateContrast(foreground, background);
    }
    
    public static int[] colorToARGB(@ColorInt int color) {
        return new int[]{Color.alpha(color), Color.red(color), Color.green(color), Color.blue(color)};
    }

    public static int[] getGradientARGB(@ColorInt int start, @ColorInt int end, @IntRange(from = 2) int step) {
        if (step <= 2) {
            return new int[]{start, end};
        }

        int[] gradient = new int[step];
        gradient[0] = start;
        gradient[step - 1] = end;

        int[] startArgb = colorToARGB(start);
        int[] endArgb = colorToARGB(end);

        int interval = step - 1;
        float[] intervalArgb = new float[]{
                (endArgb[0] - startArgb[0]) / (float) interval,
                (endArgb[1] - startArgb[1]) / (float) interval,
                (endArgb[2] - startArgb[2]) / (float) interval,
                (endArgb[3] - startArgb[3]) / (float) interval
        };

        for (int i = 1; i < interval; i++) {
            gradient[i] = Color.argb(
                    startArgb[0] + (int) (intervalArgb[0] * i),
                    startArgb[1] + (int) (intervalArgb[1] * i),
                    startArgb[2] + (int) (intervalArgb[2] * i),
                    startArgb[3] + (int) (intervalArgb[3] * i)
            );
        }

        return gradient;
    }
}

These darken and lighten methods are the exact same code. The percentage shouldn't be added in the darken method, it should be subtracted.

These darken and lighten methods are the exact same code. The percentage shouldn't be added in the darken method, it should be subtracted.

Yes, you are right. Thank you for reminding. Let me modify it later.

You can also use ColorUtils.blendARGB(originalColor, Color.WHITE, 0.2f); if you want to make a color lighter or ColorUtils.blendARGB(originalColor, Color.BLACK, 0.2f); if you want to make a color darker.

Cool! Let me check if they have the same result.