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I would like to write a simple editor of scheme to learning SICP with my iPad. So I need to highlight some characters (such as keywords and brackets) in TextView. But it seems like I could only set a global font color for it.

See and try sample of @JonB here
It uses attributedText in Objective-C

Sample based on this code

# coding: utf-8
# set attributed text dor existing textview
# see JonB code at 
#     https://github.com/jsbain/objc_hacks/blob/master/attribtext2.py
import ui
from objc_util import *

class MyView(ui.View):
    def __init__(self):
        self.width = 300
        self.height = 300
        tv=ui.TextView(flex='wh',frame=self.bounds)
        tv.delegate = self
        self.add_subview(tv)
        tv.begin_editing()

        UIColor=ObjCClass('UIColor')
        self.UIred = UIColor.redColor()
        self.UIfont=ObjCClass('UIFont').fontWithName_size_('Courier',20)

        #set up objc instance       
        self.tvo=ObjCInstance(tv)

    def textview_did_change(self, textview):
        self.stro=ObjCClass('NSMutableAttributedString').alloc().initWithString_( textview.text)
        i = 0
        st = textview.text.find('[',i)
        l = 1
        while st >= 0:                  
            self.stro.addAttribute_value_range_('NSBackgroundColor',self.UIred,NSRange(st,l))
            self.stro.addAttribute_value_range_(ObjCInstance(c_void_p.in_dll(c,'NSFontAttributeName')),self.UIfont,NSRange(st,l))
            self.setAttribs()
            i = st + 1
            st = textview.text.find('[',i)

    @on_main_thread
    #apparently this must be called on main thread for textview
    def setAttribs(self):
        self.tvo.setAllowsEditingTextAttributes_(True)
        self.tvo.setAttributedText_(self.stro)

v=MyView()
v.present('sheet')

It allows you to enter your text and it set the red color for all "[" found in it.