Each colour has a slightly different voltage although Red-Yellow are 2~2.2 and Blue/White/Green 3.0~3.2 (typically and not always depending on quality). I find the best throwie solution uses Lithium primary cells because the voltage is exactly 3.0V unlike Alkaline which ranges from 1.6 (new) to 1.2V (old) making brightness impossible to be stable and eventually insufficient voltage before the battery is fully used.
So what I suggest if you want Red use 6V with two cells in series and use 3 REDs in series and if you want the other colours use one 3V.cell with no resistors required.
The current will strongly vary with voltage in this range, so a stable battery voltage is crucial. Unfortunately Lithium cells are more expensive but worth it.
My recommendation is to buy bulk CR2 Lithium cells and choose LEDs with the highest brightness in 5mm. These cost < 25 cents and perhaps <10 cents in high volume. the battery is the biggest cost. I used to sell these to a client who wanted more than (>) 10,000 mcd or 10 Candella in 30 degree.
For each reduction in lens angle by two almost doubles the brightness and visa versa. e.g. 60 deg to 30 deg about 1.8x brighter mcd.
The 5mm LEDs have an equivalent incremental series resistance of about 15 ohms, so your husband can design the pulser circuit to switch on with less than 10% of this resistance in the electronic MOSFET switch that would be good.
Assembly time and cost is inverse with the effort put into the design. So a good design requires careful planning and perhaps a small PCB.
Not all batteries are the same cost and expect cheap ones to last less but you should be able to find CR2 Lithium cells (fatter than AA but smaller than C size) for about
$1usd each in packs of 10.
The ornament socket must be polarized like LEDs and the negative side of some LEDs has a flat edge. A socket is hard to find for LEDs but strip header sockets are perhaps the best. Since the 5mm LED pin spacing is 0.2" and the header spacing most common is 0.1" you use the outer 2 sockets in the correct orientation.
(forget button batteries, these only last a day or two depending on pulse time.)
If you need advice on your choice of LEDs let me know what you choose and ask.
Think big ornaments.