try this
String formattedText = "This is <font color='#659D32'>green</font>";
textElement.setText(Html.fromHtml(formattedText));
Or another way is using SpannableStringBuilder
String registration = "REGISTRATION";
SpannableStringBuilder spannableStringBuilder = new SpannableStringBuilder(registration);
ForegroundColorSpan colorSpannable = new ForegroundColorSpan(Color.rgb(102, 204, 0));
spannableStringBuilder.setSpan(colorSpannable, 0, registration.length()-1, Spannable.SPAN_INCLUSIVE_INCLUSIVE);
mytextview.setText(spannableStringBuilder);