Change color inside strings.xml

Try this

For red color,

<string name="hello_worldRed"><![CDATA[<b><font color=#FF0000>Hello world!</font></b>]]></string>

For blue,

<string name="hello_worldBlue"><![CDATA[<b><font color=#0000FF>Hello world!</font></b>]]></string>

In java code,

//red color text
TextView redColorTextView = (TextView)findViewById(R.id.redText);
String redString = getResources().getString(R.string.hello_worldRed)
redColorTextView.setText(Html.fromHtml(redString));

//Blue color text
TextView blueColorTextView = (TextView)findViewById(R.id.blueText);
String blueString = getResources().getString(R.string.hello_worldBlue)
blueColorTextView.setText(Html.fromHtml(blueString));

For those who want to put color in String.xml directly and don't want to use color...

example

<string name="status_stop"><font fgcolor='#FF8E8E93'>Stopped</font></string> <!--gray-->
<string name="status_running"><font fgcolor='#FF4CD964'>Running</font></string> <!--green-->
<string name="status_error"><font fgcolor='#FFFF3B30'>Error</font></string> <!--red-->

as you see there is gray, red, and green, there is 8 characters, first two for transparency and other for color.

Example

This a description of color and transparency
#   FF               FF3B30    
    Opacity          Color

Note: Put color in text in the same string.xml will not work in Android 6.0 and above

Table of opacity

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

Reference: Understanding colors in Android (6 characters)

Update: 10/OCT/2016

This function is compatible with all version of android, I didn't test in android 7.0. Use this function to get color and set in textview

Example format xml in file string and colors

<!-- /res/values/strings.xml -->
<string name="status_stop">Stopped</string>
<string name="status_running">Running</string>
<string name="status_error">Error</string>

<!-- /res/values/colors.xml -->
<color name="status_stop">#8E8E93</color>
<color name="status_running">#4CD964</color>
<color name="status_error">#FF3B30</color>

Function to get color from xml with validation for android 6.0 and above

public static int getColorWrapper(Context context, int id) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {//if actual version is >= 6.0
            return context.getColor(id);
        } else {
            //noinspection deprecation
            return context.getResources().getColor(id);
        }
    }

Example:

TextView status = (TextView)findViewById(R.id.tvstatus);
status.setTextColor(getColorWrapper(myactivity.this,R.color.status_stop));

Reference: getColor(int id) deprecated on Android 6.0 Marshmallow (API 23)

Just add your text between the font tags:

for blue color

<string name="hello_world"><font color='blue'>Hello world!</font></string>

or for red color

<string name="hello_world"><font color='red'>Hello world!</font></string>