How to remove an item for a OR'd enum?

I have an enum like:

public enum Blah
{
    RED = 2,
    BLUE = 4,
    GREEN = 8,
    YELLOW = 16
}

Blah colors = Blah.RED | Blah.BLUE | Blah.YELLOW;

How could I remove the color blue from the variable colors?

You need to & it with the ~ (complement) of 'BLUE'.

The complement operator essentially reverses or 'flips' all bits for the given data type. As such, if you use the AND operator (&) with some value (let's call that value 'X') and the complement of one or more set bits (let's call those bits Q and their complement ~Q), the statement X & ~Q clears any bits that were set in Q from X and returns the result.

So to remove or clear the BLUE bits, you use the following statement:

colorsWithoutBlue = colors & ~Blah.BLUE
colors &= ~Blah.BLUE // This one removes the bit from 'colors' itself

You can also specify multiple bits to clear, as follows:

colorsWithoutBlueOrRed = colors & ~(Blah.BLUE | Blah.RED)
colors &= ~(Blah.BLUE | Blah.RED) // This one removes both bits from 'colors' itself

or alternately...

colorsWithoutBlueOrRed = colors & ~Blah.BLUE & ~Blah.RED
colors &= ~Blah.BLUE & ~Blah.RED // This one removes both bits from 'colors' itself

So to summarize:

• X | Q sets bit(s) Q
• X & ~Q clears bit(s) Q
• ~X flips/inverts all bits in X

The other answers are correct, but to specifically remove blue from the above you would write:

colors &= ~Blah.BLUE;

And not it...............................

Blah.RED | Blah.YELLOW == 
   (Blah.RED | Blah.BLUE | Blah.YELLOW) & ~Blah.BLUE;