There are basically no pixels in an analog PAL (or NTSC) signal.
While the subcarrier is 4.43361875 MHz (or 3.579545 MHz for NTSC), the video information is still analog.
The color information just can't change as fast as the luma due to chroma being bandwidth limited to about 1.3 MHz in PAL (NTSC is different).
And the color hue information is sent as the phase of the color subcarrier, and color saturation is sent as the amplitude of the color subcarrier. So the chroma bandwidth limit just limits how fast the phase or amplitude of the color carrier can be changed.
Basically, that's why RGB is converted to YUV, and that's why the Y (luma) can have more bandwidth of 5 to 6 MHZ (depending on which PAL standard is used) than UV which is limited to 1.3 MHz before quadrature modulated with the color carrier.
You can't change the chroma subcarrier or the TV won't lock onto it.
Since 1.3 MHz color bandwidth is maximum, and your visible scanline is 52 microseconds, it basically means 135 color pixels per visible line.
Basically what it means is that you can draw sharper brightness information and apply less sharp color information. If you have analog RGB already, just add a composite encoder chip which does the conversion so you don't have to generate composite yourself.