Change the color of the Textbox font.

Step 1: Do right click on textbox. Select the textbox properties.

Step 2: Select the Color property. Select Expression.

Step 3: Put your Condition. Ex:

=iif(Sum((Fields!dow_ut_wrk_dur.Value)>20,"Blue","Yellow")

Note: Here      “iif” is similar to if statement(Conditional  statement).

Syntax: iif(Statement expression,color1,color2). If condition is true color2 will display. If condition is false, color1 will display.