Use these modes in the kernel boot parameters list with vga=0x317 for 800x600x16, more in the table:

 Colours   640x400 640x480 800x600 1024x768 1152x864 1280x1024 1600x1200
  -------+--------------------------------------------------------------
  4 bits |    ?       ?     0x302      ?        ?        ?         ?
  8 bits |  0x300   0x301   0x303    0x305    0x161    0x307     0x31C
 15 bits |    ?     0x310   0x313    0x316    0x162    0x319     0x31D
 16 bits |    ?     0x311   0x314    0x317    0x163    0x31A     0x31E
 decimal |           d785    d788     d791
 24 bits |    ?     0x312   0x315    0x318      ?      0x31B     0x31F
 decimal |           d786    d789     d792
 32 bits |    ?       ?       ?        ?      0x164      ?