## 🐛 Bug Report

When using `WebElement#getCssValue` the returned value is not necessarily a rgba value when the requested color is in fact an hexadecimal value, contrarily to what's explained in the WebElement javadoc:
https://github.com/SeleniumHQ/selenium/blob/master/java/client/src/org/openqa/selenium/WebElement.java#L246-L265

Now I've not been able to track down in the W3C WebDriver specification if the color values should be systematically rgba or not, so maybe it's just a javadoc issue.

## To Reproduce

Detailed steps to reproduce the behavior:
Request the CSS color value of an element containing an hexadecimal color.

## Expected behavior

The color is returned as an RGBA value.

## Test script or set of commands reproducing this issue

```
WebDriver driver = new FirefoxDriver();
driver.get("http://www.google.com");
// The "connection" button, cf the following gif
WebElement element = driver.findElement(By.id("gb_70"));
// expected to be RGBA according to the javadoc
String color = element.getCssValue("border-top-color");
System.out.println("Color CSS value: " + color);
driver.quit();
```

![google-button-border-color](https://user-images.githubusercontent.com/1478232/67192046-1b2c4780-f3f3-11e9-833a-ccd4b410f56a.gif)

## Environment

OS: Linux
Browser: Firefox
Browser version: 69.0.3
Browser Driver version: GeckoDriver 0.25.0
Language Bindings version: Java 3.141.59

* the children of this WebElement.
   * This method is affected by the 'implicit wait' times in force at the time of execution. When
   * implicitly waiting, this method will return as soon as there are more than 0 items in the
   * found collection, or will return an empty list if the timeout is reached.
   * <p>
   * See <a href="https:w3c.github.io/webdriver/#find-elements-from-element">W3C WebDriver specification</a>
   * for more details.
   *
   * @param by The locating mechanism to use
   * @return A list of all {@link WebElement}s, or an empty list if nothing matches.
   * @see org.openqa.selenium.By
   * @see org.openqa.selenium.WebDriver.Timeouts
   */
  @Override
  List<WebElement> findElements(By by);
  /**
   * Find the first {@link WebElement} using the given method. See the note in
   * {@link #findElements(By)} about finding via XPath.
   * This method is affected by the 'implicit wait' times in force at the time of execution.
   * The findElement(..) invocation will return a matching row, or try again repeatedly until
   * the configured timeout is reached.
   * <p>
   * findElement should not be used to look for non-present elements, use {@link #findElements(By)}
   * and assert zero length response instead.
   * <p>
   * See <a href="https:w3c.github.io/webdriver/#find-element-from-element">W3C WebDriver specification</a>
   * for more details.
   *
   * @param by The locating mechanism
   * @return The first matching element on the current context.