Make color uses a floating point representation (0 to 1). So, you're rolling a random number, which 255 our of 256 times will be one or greater, which causes the make color to max out. If you change your random range to be 0 to 1, you should see the desired effect.
When attempting to change the value of set text render color to a random value, the node will only register the maximum value in the range.
1. Open Editor (any Project)
2. Create Blueprint>Actor
3. Add Text Render Component to Components tab
4. In Event Graph, create the image attached to this report
5. Create event begin play>Enable Input set to player controller 0
6. Compile, save, close blueprint
7. Place a copy of the blueprint into level
8. Play in Editor.
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