Use spans.
Example:
final SpannableStringBuilder sb = new SpannableStringBuilder("your text here");
// Span to set text color to some RGB value
final ForegroundColorSpan fcs = new ForegroundColorSpan(Color.rgb(158, 158, 158));
// Span to make text bold
final StyleSpan bss = new StyleSpan(android.graphics.Typeface.BOLD);
// Set the text color for first 4 characters
sb.setSpan(fcs, 0, 4, Spannable.SPAN_INCLUSIVE_INCLUSIVE);
// make them also bold
sb.setSpan(bss, 0, 4, Spannable.SPAN_INCLUSIVE_INCLUSIVE);
yourTextView.setText(sb);
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