I like your thinking - I just recently assisted another user in customising the colour scheme to their own liking:
The 3-bit RGB palette gives you 8 colours to choose from, so don’t feel like you need to stick to the lower four that I chose either:
Changing the colours in the source code is pretty straight-forward - here is the part you will need to edit (around line 187 in the code):
if(dout == 3) begin //check pixel buffer data
vga_r_r <= 0;
vga_g_r <= 0;
vga_b_r <= 1; // BLUE
end
else if(dout == 2) begin
vga_r_r <= 0;
vga_g_r <= 1; // GREEN
vga_b_r <= 0;
end
else if(dout == 1) begin
vga_r_r <= 0;
vga_g_r <= 1; // GREEN
vga_b_r <= 1; // + BLUE = CYAN
end
else begin
vga_r_r <= 0; // BLACK
vga_g_r <= 0;
vga_b_r <= 0;
end
The process for then compiling the firmware is not too difficult - just means installing ‘apio’ using the ‘pip’ Python package manager, at the command line (assuming Windows) type:
pip install apio
and then, to download all the apio packages:
apio install -a
Afterwards, from the working directory (where you have both the DMG1306.v & DMG1306.pcf files) the command to compile is:
apio build --size 1k --type hx --pack vq100
This will produce a ‘hardware.bin’ file, which is only around 32K in size - so depending on your IC programmer you may or may not need to pad this file out to fill the whole 1MB chip (and be careful to take note of the orientation for the IC in its socket when removing from / inserting in to the easy_VGA board!), good luck! 
EDIT: I quietly love the idea that this kind of customisation encourages others to enthusiastically start digging down the same rabbit-hole of FPGAs that I fell in to! 
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