I tried to find a source to back up the claim that 'red' stimulates hunger. I couldn't, and in fact I found a study that suggests the exact opposite: the color red reduces food intake, because it acts like a stop signal (Genschow et al., 2012).
This also reminded me of a study by Brian Wansink and colleagues at Cornell, who had study participants consume Pringles in which every 7th chip was dyed red. They found that these people actually consumed fewer chips than controls (where none of the chips were dyed red). This may be due to the color of the chip, but another likely hypothesis is that changing colors gave participants an implicit signal of how many chips were eaten. (i.e., if all chips were red, their intake would be the same as controls)
If red/orange does stimulate hunger, it is likely a very weak effect, and highly context-specific. Therefore, asking "how much red" might not be the appropriate question to ask.
Genschow, O., Reutner, L., & Wänke, M. (2012). The color red reduces
snack food and soft drink intake. Appetite. LINK
Geier, A., Wansink, B., & Rozin, P. (2012). Red potato chips:
Segmentation cues can substantially decrease food intake. Health
Psychology, forthcoming. PDF