Most likely, this color is stored in 15 bits. If the color order is red-green-blue, that specific color would be 0x7A96.

How to convert a 24-bit color to 15-bit color:

1. Drop the last three bits of each color value
2. Insert a 0 in the first bit
3. Fill the remaining bits with the 5-bit values for red, green, and blue

In your example, it would be this:

F    7    A    5    B    5

1111 0111 1010 0101 1011 0101



1111 0___ 1010 0___ 1011 0___



0 11110 10100 10110



0111 1010 1001 0110

7    A    9    6

To convert back, split the value back into 5-bit colors and fill the spaces by duplicating the first bits of each color.
Like this:

7    A    9    6

0111 1010 1001 0110



0 11110 10100 10110



1111 0___ 1010 0___ 1011 0___



1111 0111 1010 0101 1011 0101

F    7    A    5    B    5

The colors could easily be in a different order, like blue-green-red, so keep that in mind. Also, some ROMs are byteswapped so you might have to reverse the bytes when you search and replace.

I hope this makes sense.

____________________
「きゅうり味のビールを飲めばいいよ!」
"Wouldn't Cucumber-Flavored Beer Be Nice to Drink!"

Your ROM can be named .Z64, but that doesn't mean it's in Z64 format. Try it anyways.

Or, if you're using Windows, you can use Tool64 to find out what format it's in and change it. (Look here.)

____________________
「きゅうり味のビールを飲めばいいよ!」
"Wouldn't Cucumber-Flavored Beer Be Nice to Drink!"