I use krita 2.8.7 Using KDE Development Platform 4.14.2
Opening the background option for text or the first time in a document the alpha slider is completely opaque until changing the background with the color wheel. Changing the background the shown background color always becomes this standard blue color. Moving the multiline text or editing the text makes krita show the intended color.

Reproducible: Always

Steps to Reproduce:
1.Create multiline text box
2.Write something
3.Repeat: Change Background
4.Move box/Edit text

Actual Results:  
Only after step 4 the intended background color is shown

Expected Results:  
The intended background should be shown after step 3.