To use more than 3 colours in a screen at the same time you can do something like this in the game data window. Find the palette you want to use and add a couple more RGB colours on lines underneath. For example:
PAL 2
NAME Black green
39,40,41
109,162,109
179,113,91
133,212,201
249,252,246
I have added 133,212,201 & 249,252,246 into the game data.
Then (again in the game data window) find the tile you want to change the colour of, and underneath the line that says either WAL true or WAL false add COL plus the number row of the colour you want to use - don't forget that Bitsy starts counting at 0. For example:
TIL a
00110000
00111000
11001100
11100100
01110100
00111110
00000011
00000101
NAME mistletoe 1 green
WAL true
COL
4COL 4 will give the mistletoe 1 green tile a colour of 249,252,246
When you click out of the game data window after editing the text the tiles will change to the correct colour. :)