## 🐛 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

* _not_ wait for the next page to load and the caller should verify
   * that themselves.
   * <p>
   * There are some preconditions for an element to be clicked. The
   * element must be visible and it must have a height and width
   * greater then 0.
   * <p>
   * See <a href="https:w3c.github.io/webdriver/#element-click">W3C WebDriver specification</a>
   * for more details.
   *
   * @throws StaleElementReferenceException If the element no
   *     longer exists as initially defined
   */
  void click();
  /**
   * If this current element is a form, or an element within a form, then this will be submitted to
   * the remote server. If this causes the current page to change, then this method will block until
   * the new page is loaded.
   *
   * @throws NoSuchElementException If the given element is not within a form
   */
  void submit();
  /**
   * Use this method to simulate typing into an element, which may set its value.
   * <p>
   * See <a href="https:w3c.github.io/webdriver/#element-send-keys">W3C WebDriver specification</a>
   * for more details.
   *
   * @param keysToSend character sequence to send to the element
   *