Please see your other post.  Use conditional formatting.  You can evaluate
any condition expression.  Not just the current field.

Rick B

Regarding...

Who says?
Click on the Control (in design View) and select
Format + Conditional Formatting. See below.

In Access, you would not reference a control's Text property, it would
be the control's Value property. Since the Value property is the
control's default property you do not need to explicitly state it.
You could re-write the above code like this:

Dim CAPR As String

    CAPR = Text50
    If CAPR = "1" Then
           txtSiteNum.BackColor = vbBlue
           txtSiteNum.ForeColor = vbWhite
        Else
           txtSiteNum.BackColor = vbWhite
           txtSiteNum.ForeColor = vbBlack
    End If

You can even shorten the above code:
No need to Dim CAPR as long as Text50 is a control on this form.

txtSiteNum.BackColor = Text50 = 1
txtSiteNum.ForeColor = Text50 = 1

You would place the above code in BOTH the Form's Current event as
well as the Text50 AfterUpdate event.

This will work in Single View Form View.
If you are using Continuous View Form, use Conditional Formatting.

In Design View, select [tstSiteNum].
Click Format + Conditional Formatting
Set the Condition1 to
Expression Is
In the next box write
[Text50] = 1
Set the Condition forecolor and backcolor as wanted.

Text50 appears to be a label type control, in which case you should be using
Text50.caption.  There is no Text property for a label.
In fact, you could save a little memory by getting rid of the variable CAPR
and using a direct call

If Text50.caption = "1" then . . .