If it's not a CRT monitor connected to a VGA port, most serial terminals (that haven't died just yet or spontaneously decomposed by now) have a 132 column mode. That should solve your width issue (people these days no longer design software for 80-column screens).
As for the colors, what other terminals you have defined? vt-100 was a popular standard and most physical terminals can do that. If yours can't, you are going seriously vintage here.
Edit: since you are piping a normal text console through composite, I guess 132 columns will make it hard to resolve the pixels, so the layout thing may be a hard one to get to. The width can be probably tweaked with the horizontal offset usually on the back of the monitor and the horizontal size, usually inside the monitor (careful - high-voltages lurk inside, respect your CRTs). Some monitors of the time had a switch that added a bit of the chroma signal to the luminance channel before modulating it through the electron gun, so you would have different shades of grey/amber/green for different colors, even if they all had the same luminance levels. Not sure about the Amdek (cool choice, BTW).
If none of that works, you may need to change the VGA palette itself to send luminance values that are not the default VGA ones (where luma defaults to the number of high bits on the RGBI palette). Luckily, the Linux kernel has three handy vt.default_* kernel paremeters that take each 16 1-byte values for the RGB component of each of the 16 console colors. From the kernel docs:
vt.default_blu= [VT]
Format: <blue0>,<blue1>,<blue2>,...,<blue15>
Change the default blue palette of the console.
This is a 16-member array composed of values
ranging from 0-255.
vt.default_grn= [VT]
Format: <green0>,<green1>,<green2>,...,<green15>
Change the default green palette of the console.
This is a 16-member array composed of values
ranging from 0-255.
vt.default_red= [VT]
Format: <red0>,<red1>,<red2>,...,<red15>
Change the default red palette of the console.
This is a 16-member array composed of values
ranging from 0-255.
Telling the kernel to boot with "vt.default_grn=0x00,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff" got my text console a nice green tint. You'll need to fine tune the values yourself (most of my own vintage stuff is
in Brazil, as it's horrendously expensive to ship it here, and, even if I did, I never had an cool Amdek 300, you lucky guy ;-)).