## 🐛 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

package org.openqa.selenium;
import java.util.List;
/**
 * Represents an HTML element. Generally, all interesting operations to do with interacting with a
 * page will be performed through this interface.
 * <p>
 * All method calls will do a freshness check to ensure that the element reference is still valid.
 * This essentially determines whether or not the element is still attached to the DOM. If this test
 * fails, then an {@link org.openqa.selenium.StaleElementReferenceException} is thrown, and all
 * future calls to this instance will fail.
 */
public interface WebElement extends SearchContext, TakesScreenshot {
  /**
   * Click this element. If this causes a new page to load, you
   * should discard all references to this element and any further
   * operations performed on this element will throw a
   * StaleElementReferenceException.
   * <p>
   * Note that if click() is done by sending a native event (which is
   * the default on most browsers/platforms) then the method will