We have specified some 0805 resistors to be 0.2ppm/C. On some boards/batches these are green. On others more "normal" looking. We specified Vishay. Is there some color code that indicates ultra-precision packages?
We have noticed different temperature sensitivity between the boards
There is no standard color code for the temperature coefficient of a resistor. A particular manufacturer may have a color code across their product line, or within a part of their product line. Look at the datasheets and/or consult the manufacturer directly.
Most likely the color is just a by-product of the manufacturing process, and not something deliberately selected by the manufacturer.
While all of the Vishay $$ ultra-precision 'Z' metal foil resistors I can recall buying are either robin's egg blue or molded, I can't state for sure that there are none that are not black, beige or whatever. Typically details as to appearance may not be precisely specified on the datasheet.
Your best best is probably to contact the maker (Vishay) to ask if the parts are genuine compared to the purported part number and get your PCBA supplier to confirm the exact part numbers used if you didn't specify the EXACT part number (a practice I would highly suggest you follow in future-- the next best thing to what I like doing- which is supply the critical parts for assembly plus ~5% for them to lose- costs more for sure, but there is much less question).
Measuring tempco has to be done very carefully to avoid errors from thermal EMFs. I would suggest carefully removing the part (the heated tweezer type of iron works well) and putting it in a 4-wire setup in a proper commercial environmental chamber and vary the temperature between room temperature and the two extremes specified on the datasheet such as -25°C and 75°C. The 4-wire ohmmeter (eg. 34401A must be outside of the chamber, of course, and allow sufficient warm-up and maintain temperature +/-1°C or better). If you just stick a heat gun on the resistor in open air your chances of getting a useful measurement are about zero.
Note that low value resistors have a tempco that is affected by the solder and terminations. All may be affected by PCB forces.
In addition to what Spehro says, you need to consider your source. A question which presents itself is, where did you get your resistors? If you got them from either Vishay or a reputable distributor, the color difference probably doesn't matter. If, on the other hand, you bought from a parts broker in order to save time, there is an unnervingly high chance that you got some counterfeit parts and the color difference is important. I've been involved in projects using aerospace rated (and priced) parts which got burned despite the documentation, and the sort of price differential you mention is a big invitation to the unscrupulous.
