Python 3 - on converting from ints to 'bytes' and then concatenating them (for serial transmission)

Python 3 - on converting from ints to 'bytes' and then concatenating them
(for serial transmission)

After much fruitless searching... I am having a very specific issue
understanding the way 'bytes' and hexadecimal content is handled in Python
3.2. I KNOW I'm misunderstanding, but can't seem to find the correct path.
My ultimate goal is to use the python serial module to transmit a sequence
of bytes. Some bytes are static and won't change. Others are supposed to
vary from 0-255 in value. These all need to be smushed together and
transmitted at once. (These are instructions to a programmable display.
The code contains fixed instructions to set BG colour, followed by a byte
each for R, G and B values. I am trying to cycle through colour
intensities in a loop to test, but later I'll want to be able to do this
for practical functions on the display).
A complete static transmission, tested working successfully, might be as
follows:
ser.write(b'\xAA\x11\x05\x00\x00\x00\xc3') #this works great
Similarly, I can smush them together, i.e:
ser.write(b'\xAA\x11\x05' + b'\x00\x00\x00\xc3') #also works great
Now if I want to take one of those three zero-value bytes, and replace it
with a variable, it all goes pear-shaped. After much experimentation I
ended up with something which allegedly converted the integer variable of
a For loop into a type compatible with concatenation to the above series
of bytes:
SET_BG_COLOR = b'\xAA\x03\x03'
for r in range(0,255):
red = hex(r).encode('utf-8')
blue = hex(255-r).encode('utf-8')
ser.write(SET_BG_COLOR + blue + b'\x00' + red + b'\xC3') #BGR
format
The hex(integer).encode('utf-8') was the only method so far which didn't
just throw an error about being unable to concatenate to the other stuff
I'm trying to shove down the serial connection. But it doesn't work, and
when looking at the results:
>>> x = b'\05'
>>> x
b'\x05'
>>> y = hex(5).encode('utf-8')
>>> y
b'0x5'
>>> type(x)
<class 'bytes'>
>>> type(y)
<class 'bytes'>
>>> x + y
b'\x050x5' #(this is what I get)
>>> z = b'\05'
>>> x + z
b'\x05\x05' #(this is what I want)
>>>
Looks like, although it lets me concatenate... it's a binary
representation of string data, or somesuch? So will let me concatenate but
it's not true hex values? Have I missed a blindingly obvious way to go
from x=255 to x= b'\FF'? Or is my whole approach just the wrong way to do
this? -_- thanks for your time.