The color collocation of the design scheme should meet multiple constraints such as functional semantics, visual harmony, accessibility and style consistency at the same time. The existing rule template, image extraction and condition generation methods have structural contradictions between constraint controllability and combination diversity, and the color relationship is always outside the generation process, only for post-output screening. In this paper, the problem is formulated as a sequential decision problem of discrete color combinations under multiple constraints, and the relation-aware diffusion generation framework ColorRel-Diff is proposed. The discrete color token sequence is used as the generation unit, and the mask discrete diffusion is used to make each denoising step explicitly masked discrete diffusion. The color relationship coding module is constructed, and the harmony and contrast relationships are encoded as the edge features of the relationship graph and aggregated into the relationship perception embedding through the graph attention. A constraining-guided denoising mechanism is designed to guide the sampling trajectory by the gradient of relational consistency score, and the feasibility mask is used to impose inviolable token-level restrictions on the hard constraints, and the grading softening strategy is used to deal with constraint conflicts. A three-layer inspection system of distribution level, constraint level and design availability level is constructed, and experiments and ablation are carried out on the basis of increasing constraint strictness. The results show that the proposed method occupies a Pareto-dominant position in the diversity-harmony plane, and the satisfaction rate of accessibility hard constraints remains above 95% under pressure, which is significantly better than the baseline without explicit modeling constraints. The ablation confirms that relational coding and constraint guidance dominate the gains of harmony and constraint compliance respectively.