text: Title: ColorMap.getByIndex() returns wrong colors
  Body: <!-- In the following, please describe your issue in detail! -->
  <!-- If some sections do not apply, just remove them. -->
  
  ### Short description
  <!-- This should summarize the issue. -->
  ColorMap.getByIndex() returns wrong colors.
  
  ### Code to reproduce
  <!-- Please provide a minimal working example that reproduces the issue in the code block below.
       Ideally, this should be a full example someone else could run without additional setup. -->
  
  ```python
  In [1]: import pyqtgraph as pg
  In [2]: cm = pg.ColorMap([0.0, 1.0], [(0,0,0), (255,0,0)])
  In [3]: cm.getByIndex(0)
  Out[3]: PySide6.QtGui.QColor.fromRgbF(0.000000, 0.000000, 0.000000, 0.003922)
  In [4]: cm.getByIndex(1)
  Out[4]: PySide6.QtGui.QColor.fromRgbF(0.003922, 0.000000, 0.000000, 0.003922)
  ```
  
  ### Tested environment(s)
  
   * PyQtGraph version: 0.13.2.dev0
  