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Simple Summary: Ensuring "animal welfare" and "customer satisfaction" are crucial concerns in the modern slaughterhouse. Addressing "animal welfare" includes avoiding or minimizing pre-slaughter stress as well as anxiety, pain, distress, and suffering at the time of slaughter. In general, different methods of gas stunning are used, but the conventional carbon dioxide (CO2) gas-stunning method can result in severe aversion, elevated respiratory distress, irritation of the nasal mucous membranes, hypercapnia, and lactic acidosis. To reduce these problems, sometimes inert gases (argon or nitrogen) are mixed with CO2 in different proportions. In stunning, it is a vital point that after the onset of unconsciousness, the animal never recovers consciousness. On the other hand, consumers prefer bright red-colored fresh meat. This demands consideration during the stunning and slaughtering of animals. However, different high-concentration levels of inert gases and their effects on stunning are still unknown. To support this, a trial was conducted on heme-pigment solutions (hemoglobin and myoglobin solution) in different high-concentration (80%, 85%, and 90%) levels of CO2, Ar, and N2 gas. In both cases, dark red-colored (high redness and low lightness, yellowness) solutions were found in CO2-exposed conditions, and bright red was found with N2 (high lightness, yellowness, and low redness). The effect of Ar was intermediate between CO2- and N2-treated groups. These results and concepts were applied to a rabbit-stunning method. It was proven that the rabbit started the onset of unconsciousness at a 96% concentration of N2 gas; thus, 96% N2 and 98% N2 were considered in the nitrogen-treated group. In addition, from every treatment group, 80% concentration levels were omitted. As per the results, dark red-colored blood, meat, and small intestine (high redness and low lightness, yellowness) were seen in CO2- and Ar-stunned rabbits, and a bright red color was found in N2-stunned rabbits (high lightness, yellowness, and low redness). However, consumers prefer to purchase bright red-colored fresh meat, so N2 stunning can help to satisfy their desire. Out of the two treatments, with N2, in view of the time required for stunning and color attributes, 98% N2 is better for rabbits. Therefore, 98% N2 can be considered a better option for gas stunning. The objectives of this study were to elucidate the effect of different high-concentration levels of inert gases (Ar and N2) on heme-pigment solutions and on the color of the blood, meat, and small intestine of rabbits; and to figure out the proper concentration level of inert gas (Ar or N2) for the complete stunning of the rabbit. To observe the changing of color attributes, a research study was conducted in the abattoir of the National Institute of Animal Science (NIAS), RDA, Republic of Korea. This experiment had 2 parts, (i) A trial on heme-pigment solutions (hemoglobin and myoglobin solution) was conducted in a gas chamber with different high-concentration levels of carbon dioxide, argon, nitrogen, and normal air; and (ii) a rabbit experiment was conducted—stunning with different high-concentration levels of carbon dioxide, argon, nitrogen, and the Halal method (non-stunning). A small-size digital gas chamber was used for this experiment (size: width 850 mm × depth 1350 mm). Artificial hemoglobin (Hb) and myoglobin (Mb) solutions were created from "porcine hemoglobin lyophilized powder" and "equine skeletal muscle myoglobin lyophilized powder", respectively. In the heme-pigment solutions trial, 10 treatments were used: (i) 80% carbon dioxide (T1), (ii) 85% carbon dioxide (T2), (iii) 90% carbon dioxide (T3), (iv) 80% argon (T4), (v) 85% argon (T5), (vi) 90% argon (T6), (vii) 80% nitogen (T7), (viii) 85% nitogen (T8), (ix) 90% nitogen (T9), and (x) normal air (T10). Heme-pigment solutions (both Hb and Mb) were exposed with each treatment for four separate durations of time (30 s, 1 min, 2 min, and 4 min); and every sample (Hb and Mb) was exposed during each duration of time for 10 times (n = 10). In the rabbit experiment, seven treatments were used (i) 85% carbon dioxide (T1), (ii) 90% carbon dioxide (T2), (iii) 85% argon (T3), (iv) 90% argon (T4), (v) 96% nitrogen (T5), (vi) 98% nitogen (T6), and (vii) the Halal method (non-stunning) (T7). Forty-two rabbits (mixed-breed) were collected from the nearest commercial farm and randomly selected for a treatment group (n = 6). The average body weight was 2.3 kg. For stunning, each individual rabbit was separately kept in a gas chamber, after which each specific gas was used to fulfill the desired level in the pit. After reaching the desired level of concentration, rabbits were stunned within a very short time. It was observed that the time required for stunning in the T1, T2, T3, T4, T5 and T6 treatment was 79–82, 68–73, 97–103, 88–91, 164–173, and 108–115 s, respectively. In the case of the Halal method (non-stunning), as per the rules of Islam, rabbits were slaughtered without stunning. After slaughtering, in all treatments, the blood, meat, and small intestine of each animal were collected carefully and kept in a cool room in which the temperature was −2 °C, and after 24 h, the color attributes—lightness (L*), redness (a*), and yellowness (b*)—were measured via a Chroma Meter. It was found that in both experiments (trial on heme-pigment solutions and rabbit stunning), the color values (L*, a* and b*) showed a significant difference (p < 0.05) among the treatment groups. The CO2-treated groups showed high redness (a*) and low lightness (L*) and yellowness (b*), which indicated a dark-red color, and N2-treated groups showed high lightness (L*) and yellowness (b*) and low redness (a*), which implied a bright-red color. The effect of the Ar was in between the CO2- and N2-treated groups. These phenomena were observed both in the heme-pigment solutions (Hb and Mb) and in the blood, meat, and small intestine of the rabbits. N2-stunned animals fulfill the fundamental desire of consumers to purchase bright red-colored fresh meat. Therefore, in view of color attributes, consumer satisfaction, and animal welfare, N2 gas can be thought of as a valuable alternative to stunning. Considering the time required for complete stunning and desirable color attributes, a 98% concentration of N2 is best for rabbit stunning. As such, it could be used as a better option for the gas stunning of animals. [ABSTRACT FROM AUTHOR]