使颜色在alpha通道中占80%。例如，对于红色，使用#CCFF0000:

<TextView
   ...
   android:background="#CCFF0000" />

在本例中，CC为255 * 0.8 = 204的十六进制数。注意，前两个十六进制数字是用于alpha通道的。格式为#AARRGGBB，其中AA为alpha通道，RR为红色通道，GG为绿色通道，BB为蓝色通道。

我假设20%透明意味着80%不透明。如果你的意思是另一种方式，用33代替CC，这是255 * 0.2 = 51的十六进制。

为了计算alpha透明度值的正确值，您可以遵循以下步骤:

Given a transparency percentage, for example 20%, you know the opaque percentage value is 80% (this is 100-20=80) The range for the alpha channel is 8 bits (2^8=256), meaning the range goes from 0 to 255. Project the opaque percentage into the alpha range, that is, multiply the range (255) by the percentage. In this example 255 * 0.8 = 204. Round to the nearest integer if needed. Convert the value obtained in 3., which is in base 10, to hexadecimal (base 16). You can use Google for this or any calculator. Using Google, type "204 to hexa" and it will give you the hexadecimal value. In this case it is 0xCC. Prepend the value obtained in 4. to the desired color. For example, for red, which is FF0000, you will have CCFF0000.

你可以看看Android的颜色文档。

我已经接受了三个观点。在第一个视图中，我设置了全颜色(无alpha)，在第二个视图中，我设置了半颜色(0.5 alpha)，在第三个视图中，我设置了浅色(0.2 alpha)。

你可以使用下面的代码设置任何颜色并使用alpha获取颜色:

文件activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools = "http://schemas.android.com/tools"
    android:layout_width = "match_parent"
    android:layout_height = "match_parent"
    android:gravity = "center"
    android:orientation = "vertical"
    tools:context = "com.example.temp.MainActivity" >

    <View
        android:id = "@+id/fullColorView"
        android:layout_width = "100dip"
        android:layout_height = "100dip" />

    <View
        android:id = "@+id/halfalphaColorView"
        android:layout_width = "100dip"
        android:layout_height = "100dip"
        android:layout_marginTop = "20dip" />

    <View
        android:id = "@+id/alphaColorView"
        android:layout_width = "100dip"
        android:layout_height = "100dip"
        android:layout_marginTop = "20dip" />

</LinearLayout>

文件MainActivity.java

public class MainActivity extends Activity {

    private View fullColorView, halfalphaColorView, alphaColorView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        fullColorView = (View)findViewById(R.id.fullColorView);
        halfalphaColorView = (View)findViewById(R.id.halfalphaColorView);
        alphaColorView = (View)findViewById(R.id.alphaColorView);

        fullColorView.setBackgroundColor(Color.BLUE);
        halfalphaColorView.setBackgroundColor(getColorWithAlpha(Color.BLUE, 0.5f));
        alphaColorView.setBackgroundColor(getColorWithAlpha(Color.BLUE, 0.2f));
    }


    private int getColorWithAlpha(int color, float ratio) {
        int newColor = 0;
        int alpha = Math.round(Color.alpha(color) * ratio);
        int r = Color.red(color);
        int g = Color.green(color);
        int b = Color.blue(color);
        newColor = Color.argb(alpha, r, g, b);
        return newColor;
    }
}

芬兰湾的科特林版:

private fun getColorWithAlpha(color: Int, ratio: Float): Int {
  return Color.argb(Math.round(Color.alpha(color) * ratio), Color.red(color), Color.green(color), Color.blue(color))
}

Done

现在Android Studio 3.3和更高版本提供了一个内置的功能来改变颜色的Alpha值，

只需在Android工作室编辑器中单击一种颜色，并提供Alpha值的百分比。

更多信息见下图

你可以管理颜色不透明度改变颜色定义中的前2个字符:

# 99000000

100% — FF
99% — FC
98% — FA
97% — F7
96% — F5
95% — F2
94% — F0
93% — ED
92% — EB
91% — E8

90% — E6
89% — E3
88% — E0
87% — DE
86% — DB
85% — D9
84% — D6
83% — D4
82% — D1
81% — CF

80% — CC
79% — C9
78% — C7
77% — C4
76% — C2
75% — BF
74% — BD
73% — BA
72% — B8
71% — B5

70% — B3
69% — B0
68% — AD
67% — AB
66% — A8
65% — A6
64% — A3
63% — A1
62% — 9E
61% — 9C

60% — 99
59% — 96
58% — 94
57% — 91
56% — 8F
55% — 8C
54% — 8A
53% — 87
52% — 85
51% — 82

50% — 80
49% — 7D
48% — 7A
47% — 78
46% — 75
45% — 73
44% — 70
43% — 6E
42% — 6B
41% — 69

40% — 66
39% — 63
38% — 61
37% — 5E
36% — 5C
35% — 59
34% — 57
33% — 54
32% — 52
31% — 4F

30% — 4D
29% — 4A
28% — 47
27% — 45
26% — 42
25% — 40
24% — 3D
23% — 3B
22% — 38
21% — 36

20% — 33
19% — 30
18% — 2E
17% — 2B
16% — 29
15% — 26
14% — 24
13% — 21
12% — 1F
11% — 1C

10% — 1A
9% — 17
8% — 14
7% — 12
6% — 0F
5% — 0D
4% — 0A
3% — 08
2% — 05
1% — 03
0% — 00 

黑色使用下面的代码:

<color name="black">#000000</color>

现在如果我想使用不透明度，那么你可以使用下面的代码:

 <color name="black">#99000000</color> <!-- 99 is for alpha and others pairs zero's are for R G B -->

下面是不透明度代码:所有不透明度级别都在这里

十六进制不透明度值

100% — FF
95% — F2
90% — E6
85% — D9
80% — CC
75% — BF
70% — B3
65% — A6
60% — 99
55% — 8C
50% — 80
45% — 73
40% — 66
35% — 59
30% — 4D
25% — 40
20% — 33
15% — 26
10% — 1A
5% — 0D
0% — 00

如果你总是忘记什么代码透明，那么你必须看到下面的链接，不用担心记住任何关于透明代码:-

https://github.com/duggu-hcd/TransparentColorCode

textviewHeader.setTextColor(Color.parseColor(ColorTransparentUtils.transparentColor(R.color.border_color,10)));

<TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:alpha="0.9"
        />

在Android API 11+中，Alpha范围在0(透明)和1(不透明)之间