How can I change the color of a part of a TextView?

If you have static text that needs color, you can add it without any code via the strings file:

<string name="already_have_an_account">Already have an account? <font color='#01C6DB'>Login</font></string>

then

<TextView
    android:layout_width="wrap_content"
    android:layout_height="64dp"
    android:text="@string/already_have_an_account"/>

result

I'm not sure which API versions this works on, but it doesn't work for API 19 that I've tested so far, so probably only some of the most recent API versions support this.

As @hairraisin mentioned in the comments, try using fgcolor instead of color for the font color, then it should work for lower API levels, but need more testing to be sure.

myTextView.setText(Html.fromHtml(text + "<font color=white>" + CepVizyon.getPhoneCode() + "</font><br><br>"
            + getText(R.string.currentversion) + CepVizyon.getLicenseText()));

Spannable is more flexible:

String text2 = text + CepVizyon.getPhoneCode() + "\n\n"
            + getText(R.string.currentversion) + CepVizyon.getLicenseText();

Spannable spannable = new SpannableString(text2);

spannable.setSpan(new ForegroundColorSpan(Color.WHITE), text.length(), (text + CepVizyon.getPhoneCode()).length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);

myTextView.setText(spannable, TextView.BufferType.SPANNABLE);